I think it depends on what level your at. If your just new to weights, I'd go no longer than an hour. Just don't overtrain, it's the biggest mistake i see, new guys make.
Keep in mind that it's not so much the clock that determines the effectiveness of your exercise session, but rather it's volume, balance, and intensity. How do you feel when you're done? How do you feel the next day? What progress do you make over the course of 6-weeks?
__________________
¯ Push your limits — define aggressive goals
__________·«__c u r s o r__»·
_________P R O G R E S S___P I C S
Yeha i agree with cursor......it matters what volume and the intensity you are working at. One thing i have noticed with my work outs is that my leg days take longer then my other days. Sometimes its more then an hour becuase i need more rest. Personally i try not to go over an hour though.