I also built my own pop tester using a grease gun:
http://www.peachparts.com/shopforum/showthread.php?t=123573
The manual states that they should be tested and replaced if worn. It doesn't give any time interval for replacement.
Your supposed to remove the guts and holding the bottom section pull out the injector tip 1/2 to 3/4 of the way out. If the tip pulls back in it's good. If it doesn't it's bad. It should have a good coat of diesel fuel on the parts for this test.
That's what the manual says.
Danny