I don't think there's a "should." There's how long it takes you, and however long that is, is however long it takes.
That said, boooooy, I understand being frustrated with your own speed.
For me, it takes anywhere from a week to a month, depending on how often I work on it. At least once a day is what I try for, but the thing is I have something going on with my freaking *hands*, and now they shake sometimes from the meds I'm on (and I am glad to be on them, absolutely so happy, but daaaang, that's a lousy side effect) and I can only work in short bursts, and some days, my hands are shaking too much for me to be working on anything at all. I do lots of delicate paint work, and it really suffers when my hands are acting up. And I am the sort of person, I have so many ideas exploding out of my head, it's crazy-making, and I would do one a week if I could.
Overall, anywhere from ten to thirty hours on one pony. I think the average is about fifteen or twenty, but I don't time these things. I work for the length of one album on my CD player, usually. Re-hairing, because it doesn't require super-fine detail work, goes way faster than the other parts!
If you know you can work faster if you are under a deadline, the problem may be perfectionism/tinkering. Do you work from a plan, with stuff thought up ahead of time, or do you work as you go? Because for me, drawing up a plan and sticking to it really helps me stop noodling around and get things DONE. I will sit down and say "I am going to get a base coat on this sucker today, period," and do it, and just move step by step, and not let myself get bogged down in "Oh, I should have made that more purple."
There's a phase that my work (on anything, not just ponies) often goes through where it just doesn't look like it's working/coming together, and it can be tempting at that point to tinker with it TOO much. I try to just trust my plan and my instincts and power through it.
I keep a couple of ponies going at once, though I really do like to focus on one at a time. I will, though, prep baits in great big batches, and when I am working on the details of one pony, painting THOSE, I will switch to doing a body repaint on another pony while stuff dries. So what I wind up with are ponies of various colors prepped and ready to go once I decide what I want to put on them. It's pretty handy, but if you are the sort of person who only works on one thing at a time, and it is fine if you are, then that may not be very useful to you.
Oooor it could be that you really do enjoy the process, and that's why you stay in it for so long, because when you're done, oh dear, you don't get to spend any more time with that pony. I had that going on with Stormlight (
here) and really dragged my heels on letting him go. In that case, nothing gets you over the last one like getting on the next one.
Holy cats, I armchair-psychologist-ed you. I'm sorry.
But sometimes it's helpful to think about WHY we do things, so that we can maybe interrupt that process before it begins.