Thats a bit of a loaded question - because there are two level systems for your character. One is TOTAL levels and the other is for CLASS LEVELS..Level 50 is max for each CLASS LEVEL - but if you switch classes you can keep gaining TOTAL LEVELS..
So if you switched to every class you could get a maximum TOTAL level of 750 as there are 15 classes.
I think unless you modified the game heavily it would take you several years of play to get that though.
Always remember the Tleilaxu Epigram:
Here lies a toppled god
his fall was but a small one
we did but build his pedestal
a narrow and a tall one.