Just an idea, but I think it would be a good idea to add your previous ban time to your current ban time and once it hits 2 months, your next will be perm. Example:
(Assuming 2 months is 61 days)
1st ban: Player X was banned for 3 days.
2nd ban: Player X was banned for 4 days + 3 days, 7 days.
3rd ban: Player X was banned for 10 days + 7 days, 17 days.
4th ban: Player X was banned for 6 days + 17 days, 23 days.
5th ban: Player X was banned for 15 days + 23 days, 38 days
6th ban: Player X was banned for 10 days + 38 days, 48 days.
7th ban: Player X was banned for 7 days + 48 days, 55 days.
8th ban: Player X was banned for 7 days + 55 days, 62 days.
9th ban: Player X was permabanned.
The amount of bans wouldn't matter, unless it was over 20 or something else, but rather on the amount of time banned.
Or
Instead of adding your previous ban to your current ban, your total ban time would be added and if it hits over 61 days, you get permabanned.
1st: 3 days
2nd: 15 days
3rd: 14 days
4th: 10 days
5th: 6 days
6th: 12 days
7th: 5 hours
8th: 2 days
___________
62 days and 5 hours total
9th: Permaban
-Rick