They take your bike back and give you your money back, minus how much you used it. How much they subtract is always the difficult part. After it being in for the same problem 3 times you can demand that.
http://www.ohioattorneygeneral.gov/Individuals-and-Families/Consumers/Consumer-Tips/Consumer-Tips-Auto/Lemon-Laws
The above assumes you bought it new, just realized you could have bought it used with warranty still available......