[set fair (OP)]

How often would a not too heavy cylinder need to be changed? Could they be used day after day?

[Bored chemist]

Ironically, in many hospitals you don't need an air tank.
They have breathing  quality air "on tap".

The fire service us breathing gear that's probably rather too heavy- 23Kg.
A google search tells me it gives them 47 minutes
https://www.london-fire.gov.uk/about-us/services-and-facilities/vehicles-and-equipment/breathing-apparatus-ba/

And they have put a lot of effort into optimising it, so it's unlikely that we could do much better.

However, unlike the fire service,  the medics are surrounded by perfectly good air. It just has stuff in that you need to filter out.

You can get mask/ helmets that have a battery operated fan which pulls air in through a filter then blows it out into the helmet.
This is probably the leading brand.
There are lots
https://docs.rs-online.com/af51/0900766b80ec37e5.pdf