The level required to achieve 99 in all Parameters (assuming you start at 10 in all fields, since Point-Buy alters this) is Level 267. I don't think this game will go on long enough to reach such a level, but in the interest of "why the fuck not," there isn't a real level cap to prevent you from getting there. I suppose it comes down to how much EXP Mercury wants to award the party; how fast he wants you guys to become stronger.