I've waited about 4 months between touchups ever since I've been relaxing my hair at home, which has been about ten years. I think I waited the same amount of time when I had it done at salons.
I got several shoulder-length haircuts throughout HS and college, and I don't remember needing to touchup my hair sooner because it was shorter.
I agree that you shouldn't try to wait to relax your hair if you don't feel that works for you. Many ladies here and at other boards have said that they experience breakage and styling difficulties when they wait longer than a few weeks. I think the most important factor is the difference in texture between your new-growth and the relaxed hair, regardless of overall length.