Setting up sustaining gifts that have the ability to choose fixed amount of months on a donation form can cause the page to be crowded and clunky. One good work-around to doing this is to have the donor give a regular forever sustaining gift, then manually go in and modify the duration of the gift. 

Here's the easiest way to do this:
  1. Have the donor make a monthly donation of $50
  2. Go to the sustaining gift and modify the gift duration
  3. Enter in the number of future payment she should have.  For the example of 50 total payments, it would be 49 (since the donor would have already made one payment at that point)
Once this is done, the gift will automatically be finished after the 50th payment without the donor having to do anything.