Rain or no, the chicks get fed every couple of hours. If the rain is constant the parents will pull food from their food cache for themselves and the chicks. Until the chicks are about banding age, they really can't eat much at a sitting so the cache can last quite a while. Some parents feed on a very regular schedule, some feed more in the morning, less in the afternoon and then a couple right before it gets dark, some you can't tell. But they are hardwired to feed the chicks, so the question of how long is too long really applies more when a parent has had an accident or something similar.
