In my roto league we have a 1500 IP maximum. Here's how we figured it out.
Each offensive position (we use C, 1B, 2B, 3B, SS, OF, OF, OF, UTIL) has a 162 game limit. For a MLB team, you have nine batting positions over 162 games.
So we took 162 games and multiplied by 9 (innings/game) = 1458. We round up to 1500, figuring that is pretty close to the IP you'd expect from a MLB team (works out to about 9.3 innings on average over 162 game season).
(As it turns out, we over estimated. Very few teams get that high, because sometimes they only pitch 8 innings. Last year each MLB team pitched between 1413-1464 IP.
Maybe next year I'll propose we drop down to 1450. Thanks for asking this question!)
"The game has a cleanness. If you do a good job, the numbers say so. You don't have to ask anyone or play politics. You don't have to wait for the reviews." - Sandy Koufax