town portals...how long are they really supposed to last?

I have lost so many portals over this stress test weekend its really starting to get to me, I can't even stay in town for more than 3 minutes or it will disappear... the loading screen tip says 15 minutes until a map resets so why are my portals disappearing and my maps resetting in less than that?

Since there are so many people this weekend, the portal time has been changed to 4 minutes. After the stress test weekend, it'll be changed back to 15 minutes.