I think it should be 6 and 24 so that people that play just a little a day can get a beard, but so it is still harder to max out the beard level. (So long as it is possible to set it to that)
I also would personally love to see an additional beard level for the more dedicated players, and make the system more exponential. Like: 6, 18, 48. This would allow for players that play an hour a day grow a small beard in a week and give people with a lot of free time a beard to look forward to after a few days. But I also understand that this would take more work to implement.