Most guides advise you to rest a targeted muscle for at least 48 hours before targeting again. However I've read the odd comment that Olympic lifters do squats every day.
Do legs not require the 48 hours rest, like the rest of the body does?
There are also swimmers who swim everyday (often twice a day!) does this mean you don't need to rest the upper body?
The answer is, it all depends on how you train, how you rest, and what your body can handle. Training more frequently is about being smart and having perfect programming. You can't just 'listen to your body' and go heavy until you feel like crap. You have to accept that not every session will be max effort (in fact VERY VERY few of them will be).
Squat - 595lbs -- 270kg -- Dec. 31, '09 (Provincial Record @100kg class)
Bench - 374lbs -- 170kg -- Dec 20, '08 (@100kg class)
Dead - 589lbs -- 267.5kg -- Dec 20, '08 (Provincial Record @100kg class)
Total: 1537lbs -- 697.5kg -- Dec 20, '08 (Provincial Record @ 100kg class)
Bench Only -- 358lbs -- 162.5kg -- Nov. 25, '07 (Provincial Record @ 90kg class)
Bench Only -- 376lbs -- 171kg -- Jan. 26, '08 (Provincial Record @ 100kg class)