1500 IP max over the season for all pitchers.
Weekly is untenable IMO, even if you have daily roster changes. If a good P goes on the 15-day you can't drop him and add someone else, he'll be gone when you want him back, and you'll lose those IP. If it's a yearly cap, you can make up for injuries in Aug-Sep by picking up waiver wire SP and pushing at the end. If you're heavy in IP heading into the end of the year, you can trade a SP for Elvis Andrus or something and catch up in SB. Annual limit is the way to go.
There are a couple ways around cycling pitchers.
1. Free Agent budgets. Each manager gets $100 in-season I think is the norm but you can adjust it. When your $100 in free agent money is gone, you're done with free agents, so the only way to cycle through pitchers the way some do is to get them all for a buck--impossible. Those guys in your league who make 150 transactions/year are gonna have to knock it off and draft better. Basically, if you want Joe Blow, you put him up for auction and enter your bid. He's up for bidding for 48 hrs. Highest bid gets him. If there's a tie, he goes to the player with the waiver wire priority.
2. Make your waiver claim period longer. Instead of the standard 1 day make it 3 or 4 days. 4 is the max ESPN allows. Cycling pitchers is tough if you have to wait 3-4 days to see if you got the one you claimed.