To not refresh the keys periodically is a bad idea. It is equivalent of opening up a big hole in your defences. The connection will drop after the SA lifetime expires only if there is no interesting traffic. If however interesting traffic continues to flow, a new SA will be renegotiated before the older one expires. Even after it expires, interesting traffic would trigger a new renegotiation and you should not be running into any problemsl. However, if you still need the tunnel up all the time, you could do something like what we did. Not exactly the best solution, but running a routing protocol would work where the periodic updates would keep the tunnel up.
