[Ulzgoroth]

Not an exhaustively explored subject in any GURPS sources that come to mind. Practically speaking in adventuring contexts that would usually mean 'until you need to stop playing so you can do something else'.

But barring interruptions, as far as GURPS rules go it's down to fatigue. Fatigue costs for most activities are not detailed, because it's not really expected to come up - but you definitely want to look at Basic p426 for some points of reference.

I'd say playing an instrument might be around the same exertion level as hiking if you're not doing anything too challenging, and potentially as hard as paced running in more intense cases. I've only got a little musical experience, though.