We have the following policy: "snap sched 2 6 12", 2 weeklies, 6 nightlys, and 12 hourlies.
Interesting. I use "snap sched 1 6 7@8,10,12,14,16,18,20". This tends to translate to about a 10 - 20% snapshot reserve depending on our filer's usage pattern... I like to keep that snapshot reserve 50 - 75% full most of the time so I have a buffer to handle large deletes gracefully when they occur. What snap reserve do you use? I am curious to hear from other admins on this issue as well.
Someone else mentioned that when both the filesystem and snapshots are full, it can be very tricky and may require manually deleteing snapshots. This is quite true. As I think the manual mentions, "preventing" a snapshot from growing could lead to strange behavior (i.e. rm *failing* due to lack of disk space). The only real workaround is to keep a snap reserve large enough for your desired schedule. If you need to, buy more disk. :)
Bruce