I play at 3 screens and up to 12-14 tables, 15-16 is quite difficult. But i can afford it for 3-4 hours. and it's not difficult! The more tables you play - the easier the game is. All you have to do is to play tight and act at the table where the good hand is, rest you're folding... so if you have 12+ tables then you're always playing 1-4
hands at the same time. But it's person dependent i guess, some people have good attention and can make fast decisions, others need time... After all - it comes with practice. Just start it and you'll love the rhythm, and keep in mind that you can always fold, stop, close and go fishing at the nearest lake instead of crazy poker bet\fold marathon.