You can't order games for a set amount of time.
Please note this chart:

1 game out at a time-$15.95 per month
2 games out at a time-$22.95 per month
3 games out at a time-$29.95 per month
4 games out at a time-$36.95 per month
Members are not able to enroll in the 3 and 4 game plans until after they have had a paid account on the site for 60 days.

You will keep getting charged for Gamefly until you cancel the service.