I have a bit of a different answer to this one. How about waiting until your training hits a "plateau"? IMO juice should only be used when natural training is failing to produce the results you're looking for.
Train hard...when your hard work stops eliciting the gains you're accustom to, change your diet, training regimen, etc. That should get you over the hump. Then when you hit another "plateau" change it all up again. When you've done that, and STILL aren't getting the results you want...THEN hop back on the sauce. I don't think TIME should be the dependant factor, it should be based upon results of your current training.
That being said, I'm gonna add a link to an OLD post of mine that kinda details when I think you should cycle (hypertrophy phase).
BD's old thread
Hope it helps...