That'll depend on the amount of experience needed per level more so than the actual amount of levels.
It'll mainly be dependent of their long-term plans for the game. How many expansions do they want to introduce, what level do they want to attain at maximum. What figure sounds not too daunting for newer players who join in late? Back in the old days of MMO's games tended to cap out at 99, 120, 150, or even higher. For newer players this sounded like a steep climb to make, just to get to the fun. And games tended to be left aside.
Just for that I wouldn't be surprised to see them go by increments of 5. Ensuring that e.g. 8 years from now the level cap is not much higher than 75 or 80.
