Setting realistic goals

I am hoping to drop some time this summer by joining a summer swim program, which by the way...if anyone knows of good programs I'm still looking :) , but hopefully in Texas. Overall I need my times to go down about 30 to 40 seconds but I realize that is not going to happen in one summer. How much is a realistic goal to drop in one summer? I was hoping that maybe 10 to 20 seconds would be nice, or is that too much? Thanx a bunch!!! ~Kyra