You may leave Rookgaard as soon as you have reached level 8. To do so, go to the oracle in the village.
If you leave Rookgaard with level 9 or above, you will find that you gain fewer hit points, mana, skills etc. than you would have received if you had left Rookgaard with level 8. These reduced values will not be readjusted!
Also, please note that it is impossible to return to Rookgaard unless a character dies often enough so that the level is reduced to 6. If this happens, characters are completely reset. This means they will reappear on Rookgaard with level 1. They will lose all of their equipment and carried items as well as any levels, skills, spells, and the money in their bank account.