Choosing an interval for the cronjob.

You can use any time interval you like for the cronjob, but keep in mind that the time interval should be at least as small as the smallest snapshot interval that you use. E.g. if you want to create hourly snapshots the cronjob should run vhpi at least every hour, otherwise you won't get a snapshot for each hour. You should also keep in mind that the more frequently vhpi is run by your cronjob, the higher is the chance you get a new backup. E.g. if you use a cronjob that only starts every 24 hours, chances are high that you won't get a backup for several days in a row, because your client machines might be offline at the particular time your cronjob fires. So even if your smallest snapshot is supposed to happen daily, you should consider making the cronjob run vhpi each hour or so. That way chances are higher that you get a daily backup.

TL;DR Just use an hourly cronjob.