The highest possible rating is infinite based on the calculation, however the amount of necessary work reaches an amount that is unmanageable. To give you some basic idea. To go from rep 28-43 it takes 10x the raw reputation points that it takes to get from rep 25-28. At that point it gets to where it takes about double the work (you have already done cumulatively) to reach 9 more levels.

1000000000 = level 25
10000000000 = level 28
100000000000 = level 43
1000000000000 = level 52
10000000000000 = level 61
100000000000000 = level 70

At this point you double your overall work to reach 9 levels higher. So a person can gain an infinite amount of reputation points (unless there is a cap) However, for a person who has a level of 70, it is going to take a total of double the amount of work he has already put in up to this date to reach level 79