When he can run 13:0x regularly, or when he can run 3:4x for the mile, or when he can run sub 3:35 in any and all types of races.
And if he could do the above, he would be the first American to turn pro who was actually ready to run pro.
Aside from the money aspect, what's the point of going pro with a 13:25 PR when you're looking at a couple years of hard work and good luck to become competitive enough to be a factor in any race? I don't think it benefits a US based runner to go pro if they are only competitive against US based runners. Because the best competition in the US right now is in the NCAA. When German is ready to be competitive in European meets and senior WCs/Olympics, that's the time to go pro.
The money aspect is important but I think being patient may lead to a bigger contract. If Rupp went pro a couple years ago would his contract be the same as it would today? There's quite a difference between two years ago when he was a very good runner who "might make the WC team" (which he did, but nobody would've been surprised if he didn't) versus today where he is a huge star, was the person expected to win the WC trials, multiple NCAA titleist and record holder, and looks unstoppable at any distance from 3000m up (with quite a few milers probably worried when he shows up on the line).
German may be all these things next year, but right now he is just an extraordinary young guy who can get 5th in our national championships. If I were a betting man, I would say next year he makes the 13:15 range and wins another NCAA title or two but stays in the NCAA until 2011 where he crushes records, makes the 13:05 range, makes the WC team with people talking "medal threat" and THEN goes pro with the biggest contract yet for a distance runner.
Webb and Jager went pro as much for the coaching change as anything else. Their situations were different.
Lukezic, at least at this point, looks to have wisely capitalized on his hype to get a good contract that would not have gotten astronomically better had he stayed in the NCAA.