it depends on so many factors.
if you're a sedentary individual, chances are you can lift a lot more than if you have a very active lifestyle. people who work at a computer and are on their asses all day are going to be able to burn themselves out more in the gym than people who haul crates or chop down trees for a living.
when i worked at the steel mill, more than a 45 minute workout was enough to KO me if i did more than 4 a week. in school, where my day mostly consisted of sitting in class and sitting at my computer to do work, lifting for 90 minutes wasn't a problem at all as long as i kept eating enough. considering i do zero physical exertion outside of playing with my dog a little bit, the 6.2% of my day that's actual exercise can't possibly overtrain me.
it's totally individual. once again, GENETICS are a huge part of it. you can't really know unless you amp up your workouts and back off when they seem to be too much.
ever wondered how those russian olympic lifters can lift for 4 hours a day? it's not just because they're going sub-maximal, it's because that their entire day is that workout. the rest of the day is eating and recovering. top level bodybuilders, even the non-geared ones, can pull two a days at the gym thanks to not taxing their bodies ever, save the training. my stepdad runs triathlons. he lifts weights 4 days a week, along with running, swimming, and biking. it's not uncommon for him to be in the gym for 3+ hours. common wisdom would say he's overtraining, but he doesn't.
CNS burnout is a possibility but all you have to do to avoid THAT is not do heavy singles week in and week out. i made that mistake and my joints got pissed off at me, but that's not "overtraining".