It really depends on your speed and the level of detail you want. Painting a whole army can seem like a big or even impossible task especially if you have all the models already but it just takes a lot of patience.
For me personally, on a solo figure that I want to look real good, like an army leader or someone special like that, it would take me a couple of hours. What a lot of people find works best for a lot of infantry is an assembly line method. Do 1 squad or about 10 guys at a time. Line em up, plan what colors you want to use. If your uniform is say, blue & yellow, start with your blue color and paint it on each guy, then go back and do whatever blue shading/highlights you need. Then do all the yellow. 1 color at a time each time till they're done. Once you get your technique down you could do those 10 guys in one evening if you set your mind to it.
I tend to devote 1 night a week to building & painting, since I have a lot of other stuff going on. So setting a little schedule for yourself and planning ahead might help a bit too. Then you're not constantly painting and getting burned out before you're finished. You might even find yourself looking forward to your "painting night" especially if it's after a long boring day at work/school