It doesn't matter how much time you can invest, there's simply no point to it. Your body grows muscles while you're resting, as long as there's a sufficient impulse. Maintaining that impulse does not require a lot of time.
The reason many strength programs are not 5-day programs is not because most people don't have enough time, but because it can actually be more effective to train less. And I do not mean more effective per time investment (that would be more efficient), but more effective overall.
When you train what you do is you strain your muscles to the extent that your body needs to repair them (just like a small wound). This repair process overcompensates so that your muscles are larger and stronger than before. The repair process is what grows your muscles, and it takes several days. Now of course you don't have to wait for the repair to finish completely (otherwise you couldn't train more than once a week or so), but you need to wait long enough to make sure that muscle growth is maximized. If you continue to damage your muscles too early during the repair, you risk losing strength instead of gaining it, and your risk of injury increases.