Sage Advice gives us a guidance on how to know if a spell can be dispelled or not:

Can you use dispel magic on the creations of a spell like animate dead or affect those creations with antimagic field?

Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend.

Wish does have instantaneous duration, so it is not questionable that its effects cannot be dispelled, except one:

You grant up to ten creatures you can see immunity to a single spell or other magical effect for 8 hours.

Strangely, this is the only effect offered by wish that includes duration.

Does this mean that if I use wish to gain this effect, it can be dispelled?

No. The duration of Wish is instantaneous

In the quoted Sage Advice, "duration" refers to the the "Duration" field of the spell's description, not whether the spell has a lasting effect. Wish has a duration of Instantaneous; therefore, the spell is instantaneous, even though it has a lasting effect.

I realize the example you give involves an ongoing effect created by magic. When someone tries to dispel it an hour later, though, the catch is that the effect was created by magic. After it was created, the effect simply existed, without the aid of ongoing, dispellable magic.

As the Player's Handbook puts it:

Duration

A spell's duration is the length of time the spell persists...

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't
be dispelled, because its magic exists only for an instant (PHB p.
203).

Compare this to Plant Growth. It can either cause plants to become overgrown or cause soil to become enriched for one year (depending on how you cast it). The spell's duration, though, is Instantaneous. It cannot be dispelled.

• The overgrown plants are simply overgrown. If you want them gone, get
  a lawnmower.


• The enriched land is simply enriched. If you want it un-enriched,
  wait a year. If you can't wait a year, you'll have to get creative. Magically-induced erosion could work, or you might let the land stay enriched but kill the plants and/or the farmers.

Also compare to Fireball, which can burn people and set things on fire. The spell is instantaneous; it cannot be dispelled.

• If it sets your sofa on fire, your sofa is now covered in nonmagical fire; use a fire extinguisher.


• If it injures your fighter, the burns are real, but there's no magical residue; use Cure Wounds.

The bottom line

It's tempting to look at a spell like Wish or Plant Growth and say that, because the effect has a duration, the spell must have a duration. This is not what the rules say, though.

Fireball has a duration: The magic happens instantaneously. Fireball's effects also have a duration: they last either permanently or until someone reverses them (with a fire extinguisher, with healing, etc.). According to the rules as written, the addition of a timer to the effect of Wish does NOT mean that the effect continues to be magical. Wish CANNOT be affected by Dispel Magic; there's no ongoing magic to dispel.

This is unclear; ask your DM

As you noted, wish has an instantaneous duration. There are two ways to interpret this effect in that light:

1. This is an instantaneous effect that lasts for 8 hours - Not dispellable

This is a weird one, but it could theoretically be that the spell and its magic are instantaneous but that the protection it offers lasts for 8 hours. This would be an odd interpretation to say the least given that the vast majority of spells' effects follow their duration (see plant growth discussed in the last section).

If you were to side with this reading, it would mean that you could not dispel the effect since the spell that triggered it was instantaneous and there is no longer anything to dispel.

2. The effect extends the duration of the spell - Dispellable

In this reading, this specific effect has its own duration separate from the duration of the spell and could be seen to override that duration. Again, I don't know of any spells that do this so it is a weird edge case. However, if you read it this way it means you would have the effects of a 9th level wish on you for 8 hours and that a 9th level dispel magic could remove it.

Neither is obviously better

Honestly, neither option seems significantly more compelling than the other mechanically. For what it is worth, plant growth also has wording similar to this, but it also has the exact same ambiguities in that the wording can be interpreted in either of the two above ways.

Out of them, #2 is a bit more intuitive though to me in that it just extends the duration of the instantaneous magical effect into an 8-hour-long magical effect. On the other hand, #1 adheres most strongly to the spell as written (following the duration as listed).

Being that this is ambiguous, your DM will have to decide which is the best option for the table.

Yes, it is a spell effect and can be dispelled

A number of spells can create magical effects that last beyond the durations given for those spells. Most such spells allow the caster to cast the spell repeatedly every day for some period of time in order to make the effect permanent. Generally, these spells say that the effect, rather than the spell's duration, becomes permanent. There is a Sage Advice answer from Jeremy Crawford clarifying that such effects can still be dispelled:

If the effect of a spell becomes permanent, it can be dispelled, unless its description says otherwise.

The effect of wish in question is not permanent, but rather lasts 8 hours. However, I believe we can infer that the above ruling about permanent effects is an instance of more general principle: if the magical effect of a spell lasts longer than the given duration of that spell, it is still considered a magical effect of the spell, so it can be dispelled, unless its description says otherwise.

In other words, the duration given describes how long the spell normally lasts, but it doesn't override the text of the spell if that text describes effects that can last longer, and it doesn't mean that such effects are not still considered ongoing spell effects that can be dispelled.