You can modify the grace period for filesystems using equota:

# edquota -t

This puts you in an editor of a temporary file that looks like this:

Grace period before enforcing soft limits for users:
Time units may be: days, hours, minutes, or seconds
Filesystem             Block grace period     Inode grace period
/dev/hdc1                     7days                  7days

The text in the file is nicely explanatory. Be sure to leave your users enough time to receive their warning e-mail and find some files to delete!