depends on how hard you hit it for one thing....but id say 5-6 weeks at the MOST
of course you dont have to take a week all the way out of the gym....you can come in and do some relaxed stuff, or even come in and do some actual lifts but with really low volume
this is a great article on this very subject
http://www.t-nation.com/readTopic.do?id=546953
IMO it is false for people to think they are really doing something based on how long they go without a break......like there is a guy in my gym...he told me the other day "I am going 8 weeks without a break"....but he isnt making great gains...IMO he uses way too much intensity (forced reps, drop sets etc)....he could probably go 3 weeks with the 4th week "backed way off" and make decent gains....but people just cant see that..they think they will shrink
The basic Russian approach is generally 3 weeks of hard training and the 4th week is almost always a "deload" type of week.....also, WITHIN the 3 weeks the volume and intensity vary...they dont just go balls to the wall each workout...some workouts are really intense, some are really high volume...but then some are just "average"
also, it could depend on your routine....if you are doing something like each bp once per week and you are training on M,W,F and you arent taking many sets to failure, then you can go longer without needing a break
but if you are doing a 6 day, 3 way split (each bp twice per week) and you are pushing alot of volume or intensity, then you probably should go 3 weeks or so, and then deload for a week