We usually have at least 3-4 days. And if you've been around long enough, you kind of know that something will be coming -- for instance, when the notice came out tonight about practice changes next weekend, it was no surprise that something would change because of JO Nationals.
We don't get refunds for cancelled practices, but we don't get charged if an extra practice gets added for the boys. (Doesn't happen on the girls' side, though sometimes a practice time is changed.) If a girl or boy has to miss a scheduled practice, the coaches are usually OK with the kid coming in on a different day if there's a suitable group with which s/he can work.