Prior's being targeted for a mid-May return (as of yesterday anyways). It seems like the timeline could change any day. They're bringing him along slowly, which might turn out to be a good thing - you would probably rather not see 'em rush him back too soon.
By the same token, even if Prior is determined "ready" he'll probably pitch two or three complete-games with 10+ k's, then he'll go out for the next start and strain his elbow again. He's always that kind of risk. My best guess is that he'll give you 15-20 starts and hit the DL a few times. I can't count on that kind of guy if I want to make a play-off run. My plan is to sell high on Prior after he returns and throws a few nice games, maybe deal him for a slightly comparable pitcher who I can count on to help me in the playoffs.
As a disclaimer, I live in MN so I'm not quite as knowledgeable about it as a die-hard Cubs' fan might be. It's funny though, if you go on the Cubs mlb.com fan forum you'll see some pretty hilarious rants about Prior that turn into heated debates between fans. That's usually how I get my Prior updates.