You should stay with it until you are done with school. For every completed degree level, i.e. Undergrad, Master and Doctorate, College Plus will award you 10K bonuses respectively. So if you are starting out as a freshman, you have a potential of 30K miles when you get your Ph.D.
You'll have to mail MP your final transcript or something upon degree completion. Check out their website for details.
You can also start with the no-fee College Plus Visa card. The sign-up bonus miles are not as good as the regular MP Visa but will be a good way to start building your own credit history.
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