If you use a mind read spell on a rebellious slave there will be a strength value which shows how much the slave is rebelling. This notionally is the number of days in jail before the slave stops rebelling, however there are so many modifiers to this from jailers, fear, etc that it's just a guide. Spells and potions can help. Low grade slaves rebel less so if you want a fast turnaround stick to low grade slaves at first.
