I do a unit at a time, with no limit like "done in a day" or whatever. Just start when I feel like, and finish when I need a break. Usually, though, I can get a unit done in one sitting, by painting efficiently.
This is what I do:
Assemble and prime everything I'm planning to paint, be it a unit, character, or vehicle. I try to do this a few days before, or the night before I plan to set aside some time for painting, because I like to let the glue and primer set before I start. If you have to though, you can dive in once the primer is dry.
After that, I basecoat. I pick a single color, and do it on all the models in the squad. So like, with 10 orks, I'll paint the skin on all the models before laying any other colors down. Then, I paint all the armor whatever color, then all the leather, all the guns, etc. I let myself be sloppy at this stage, not worrying too much if some color bleeds off into an area its not supposed to be.
By the time I am done putting the coat on the last model, the first one is dry, so I do a quick touchup, then start highlighting. Again, 1 color on all the models, then the next, and so on. I try to be a little more careful here, making sure everything goes down where its supposed to be.
Then, repeat the highlight stage with a brighter shade.
Next comes shading. I use the GW washes, but you can mix your own if you want. I have a bottle of armor wash that I made that really works well making metal look kinda dingy and worn - equal parts devlan mud and asurmen blue, with a couple of drops of scorched brown thrown in, and thinned a little with water. I mix up a big batch when i buy the bottles, so i have it handy.
So for orcs, I use my armor wash, thraka green, and devlan mud (aka, the best wash evar) I stick with doing 1 color per model till they're all done, but it goes really quick because I just slop the washes on in their appropriate places. I'm a little careful with the green, cause I want it only on the flesh, but the armor wash and devlan mud tend to look good over almost anything, so I'm not careful at all. If a little gets on the flesh, it tends to just deepen the shade a little, or make it look dirty or grungy, which isn't a bad thing for orks, anyway.
After shading, I do detail, same process. I do all the eyes, then all the eye highlights, etc, any tattoos or facepaint, and rust on the metal.
It might not seem like it saves all that much time doing it this way, but I've found that you save a lot by not having to wash your brushes as often, remix shades because they've dried out while you've been working with another color, etc.
It's rare that it takes me more than one sitting to do a character or a squad. My stuff won't win any golden daemons, but it looks damn good on the table.
Hope that helps, and wasn't too much to absorb!