As an assistant, you are usually engaged for six years, during which you split your time in preparing your PhD and providing services to the faculty (usually a teaching assignment). Although it will take you longer to obtain your PhD than with other types of financing, this way you are given the opportunity to make different skills your own: research, teaching, providing services, ... Such diversification always comes in handy once you have obtained your PhD and re-enter the labour market.
As a PhD candidate you can apply for assistant mandate vacancies, so be sure to regularly check our vacancy page. You do not necessarily have to have excellent grades in order to be eligible for an assistant mandate: for different PhD projects the emphasis is different and therefore they look for different profiles. Of course it is required that you are passionate about research and that, in addition to that, you enjoy your qualities being put to use in the education process of your faculty.