Normally when you update a feed your software or system will send out a ping to the service provider, like Google. (You can also do this manually.) That ping is a signal to them that the content has been updated and that they should fetch your updated RSS feed. Without the ping most feed aggregation services will check on a regular basis. Google, for example, seems to drop by my feeds once a day if I don't ping them. There may be greater or lesser frequency depending on how often you update.