Both are bad. But asking too early is worse. You risk injuring the relationship by signaling that you value the donor’s money more than the donor. And sometimes that is never repaired fully.
When you ask too late you risk getting a smaller gift (you didn’t strike while the iron was hot). And you risk appearing as though you are less than professional. Shrewd donors expect to be solicited.
So when is the perfect time to ask? When the donor lets you know she is ready to be asked.
Hint: If this answer seems unsatisfying and you can’t recall when a donor has signaled her readiness to be asked, you have been asking too soon.