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In the Tyranny of Dragons campaigns, there are magic items called Dragon Masks.

They also appear in this supplement (p. 4).

In particular, I'm interested to know how this property works:

Dragon Sight. You gain darkvision out to 60 feet, or to an additional 60 feet if you already have that sense. Once per day, you can gain blindsight out to 30 feet for 5 minutes.

So since you can only do it once a day and it only lasts 5 minutes, it is something you must activate, right? But it doesn't explain how. In particular, what is the action economy cost for this? Does it cost an action? Is it an object interaction? Do you "just do it" like with a fighter's Action Surge?

I'm hoping there's something I've missed, or perhaps some generic rule somewhere that says something along the lines of "if it doesn't say, it's an action" or "if it doesn't say, you just do it".

There are rules for activating magic items (here) but as you have possibly already noticed, they just say that the description of each item details how an item is activated.

There doesn't appear to be any 'default' rule, so in the case of missing details I would say that it is up to the DM.

In the case of the Dragon Mask that give blindsight for 5 minutes, I would probably rule that it's just a free action since this is a unique powerful object and the blindsight is an ongoing and long-lasting personal 'condition' effect (as opposed to an attack or spell etc). So I doubt it would be unbalancing to say the wearer can just activate it with a mere thought.