Why, and for how long? And how did you decide when to stop?

My son is 13 months old and I'm still pumping twice a day at work - he still nurses on demand when we're together. I'm very fortunate to have a private, locking office and a supportive boss who would never tell me to stop. I don't look forward to pumping, but I don't dread it either. It would be nice to have the extra time at work so that I didn't have to bring so much home, but I'm not sure that's a good enough reason to stop and I really want to keep my supply up for as long as possible.

Am I crazy to keep doing this? Any other long-term pumpers out there?