If you only heal yourself and your team members up to full and no more your mana return will be limited. Overhealing in the spirit realm is a really good thing!
I'm gonna play devil's advocate on this one, I always leave the moment I have gotten everyone up, and haven't had any problems. The buff you get essentially allows you to spam Flash Heal till it expires and still be at full mana at the end of it. Plus if you make sure to alternate the two healers, so that noone ends up staying out too long, you literally cannot get your mana low enough to worry about it.
That buff is in the order of 15k mana per second. By comparison, a spirit focusing disc priest, including all buffs, gets at most 6-7k mana per second from normal means. Other specs are closer to 4-5k.
My point is by the time you get yourself up, the buff you get should be enough to keep you going till the totem after next, more of the buff would literally be wasted. I think getting out fast so the other healer can be relieved is more important.