Lifting just once a week is enough to reap some benefit, according to coaches. Not only is strength training more beneficial to overall fitness than most people realize, but you don't even need to spend hours in the gym every week to reap those benefits. Almost any amount is much better than nothing. While greater effort will produce better results, returns decline rapidly.
Just one or two half-hour sessions per week can get most of the results you'd get two to three times that amount of investment (and that's a deliberately conservative estimate). This generally applies to any form of exercise, but especially with strength training. In a world where virtually everything related to health and fitness is controversial, this is a fairly established science.