That is the exact reason why I worked construction building new high end houses...I wanted to learn how to build correctly. Anyone can build something, but to do it right is another story. I added an addition onto the back of my house 3 years ago, it took me a couple of hours to frame, sheet and stand the walls. I had the entire room enclosed and under roof within 2 days.
I would say that after the concrete work is done, I could have a garage built in less than a week. There is a lot of hints,tips, and shortcuts to make building easier on yourself.
If you need any help or pointers, shoot me a PM and I will do the best that I can to help out.
On a side note, you are pretty lucky to even get a contractor to actually show up to give you an estimate. Most of the time they either A) don't even show up for the estimate or B) give you an estimate but never show to do the work or C) Price gouge, do shoddy work and take about 10x longer to finish than what they say.
I would looooove to get into a business of my own building garages and sheds only. If I could figure out how to go about starting it, I'd be in like Flynn.