As a past employee of Hallmark/Crayola, I've had a bit of experience with Model Magic and can offer what I know:
It depends on the thickness of the Model Magic. It could take anywhere from 3 hours to 3 days to dry completely. If you wanted, you could experiment with using a hair dryer. I've never tried it, but I'm fairly certain it wouldn't hurt anything.
A word of warning since you're concerned about it being lumpy: Model Magic is near impossible to sand (if anybody out there knows of a way to make it work, however, please please share!). My friend made her Homestuck horns out of the stuff and had me sand them for her, but they kind of flaked apart and became almost suede-like in texture. Don't get me wrong, I'm not trying to bash on Model Magic. It's awesome stuff, but like any air-dry modeling clay, it's frustrating to work with unless you move very quickly (Model Magic in particular starts drying and cracking within minutes).
Your best bet for a smooth, uniform surface is sanded Sculpey or another oven-bake clay. If you have the means to procure some, absolutely do! It's quick and efficient and I've never had a bad experience with it.
I hope this helps!
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