So you probably burned out a key Portal for your advance at least once. I did it a lot and then used all that data to decode logic behind it, and here is how to not burn them out!
So, when you hack a portal, portal goes to Cool Down period. That period is exactly 5 minutes or 300s. If you try to hack it before that period it will send you error message with additional time that you should wait to hack again. If you hack portal more than four times in the range of 4 hours it will go out to Burned Out state! Oh NO what I have done… but wait, there are tricks to it too.
First off, when Ingress portal go to Burn Out state it will stay like that until end of that 4 hour cycle. So if you burned it out close to end of that cycle, portal will be back soon. If you are too pushy it will be burn out for almost all cycle. We are still not sure if cycle starts with your first hack, or the day is already divided on 6 cycles x 4 hours. We will update this info when we decode its logic.
TIP: A WRONG STRATEGY! Lot of players are burning out portals intentionally, so that other players can’t hack it. This is wrong! Portal is burned out only for you, so there is no point in doing so. After all you are using your precious exotic matter (XM). So this is a bad strategy, and avoid doing it!