IMHO, A hobbyist should
arrive on time, and also
leave on time. At the least, be considerate of the time!
Now - HOBBYIST BEWARE - If I'm super horny and you haven't given me MY orgasm, consider yourself fucked. I am going to chain you to my bed and feed you nothing but water and aphrodisiacs until you give me a nut! Rawr!
Haha, just kidding.
But seriously though, it's not that anyone has to be a clock watcher, but both parties should have a general sense of time and it's boundaries.
No one wants to have their time rushed, and no one wants to have their time taken advantage of.
Also, like the old saying goes, "Don't wear out your welcome!"
Oh, and as far as etiquette is concerned, I never ever upsell time in a session to begin with, and if a gentleman desires more time, then you should let him know what your expectations are. Simple.
It's not that complicated, but it should always be polite and respectful.