The first one I did was a 308 and it took a few nights after work, mainly because I ran into a few 308-specific snags I didn't know about. The next one (12ga) I did this weekend in a couple hours while watching a football game. I was apprehensive at first, not knowing anything about AK internals and not having any gunsmithing tools, but after you dive in it's fun, you get to know the "guts" of your rifle, and get the satisfaction of doing it yourself.
Summary:
Remove buttstock, receiver cover, recoil spring, bolt/gas piston
Remove "guts" (fire control group and hammer assembly, 2 main parts) by punching out or drilling out a couple pins (depends on model)
Remove "trigger plate" by drilling out a couple rivets on the bottom of the receiver, most models have the holes for the regular trigger & pistol grip already there under that trigger plate, revealing the true form of the rifle, an AK
Bolt on pistol grip, new trigger guard, put new fire control group and hammer in re-using the pins you puched out earlier, a couple springs, AK-style buttstock, and voila!
The cost of conversion parts is between $130-$175 for a fixed stock, about $100 more if you want a folding stock. I second the recommendation for Dinzag. Brian is a great help and can give you a list of the parts you'll need and the costs. There aren't really any complete conversion "kits" because you have several choices for buttstocks, pistol grips, fire control groups, and trigger guards, but it's fun to customize it to your tastes.