BE CAREFUL: I'M USING AFTER EFFECTS CS4 ('coz I'm broke). YOUR WINDOWS MAY VARY TO MINE (I hope it doesn't). YOU'VE BEEN WARNED.
When dealing with long compositions, you could always resize your Work Area and Trim Comp to Work Area, making your animations shorter but when you're not careful, it might cut some animation either in the beginning or the end of the comp.
Since I don't know if you've found your way out of things, pictures are provided
You could also do this on Render Settings on the Render Queue Tab > Render Settings > Time Sampling Box > The "Custom" button at the right of the Start, End, Duration Times
You could also reduce your FPS (frames per second) in the Render Settings at the Render Queue Tab (you need to add the comp to the render queue) but when you'll render these into a full video, you might notice constant stutters as if it was laggy.
You could also change it in the Composition Settings in the Composition > Composition Settings albeit a more destructive method
Mix these things together, and you'll reduce your rendered images even more. Be careful though.
I'm new here and I don't know the standards of answering questions. I hope this helps you and good luck with your project!