I know. it's tough to make line infantry worthy of comment after the first batch
For 40k, yes painting up infantry is always a grating job. Doing them assembly line helps immensely, but I've spaced out my Marine infantry considerably. What makes it so disheartening is that even when you finish 10 guys you don't have anywhere near a usable force. Basically it was shear force of will that got me through the 30 infantry for my marine army and thinking about the conversions I'd do once I was done
Epic I find easier because of a couple things. To start with, it's quick to paint up and it goes a lot further. For my marines, painting up 8 stands of devastators and a thunderhawk is 700 points toward a 3k army and can be done in a weekend to a pretty high standard. It makes you feel much more inclined to paint things up when you can see real progress towards fielding a painted army.
Another example would be these necrons. with the three phalanxes, 6 tomb spiders, 2 pariahs, and the pylon, I'm over halfway finished with the army for what is effectively a months worth of painting only a couple hours here or there.
I do have to say, that after finishing up half a chapter worth of epic marines, I am getting a little tired of painting little yellow men!