I always did it by installment, but that meant that I'd end up having more money some terms than others.
Loan, minus rent, minus textbooks divided by the amount of weeks until next loan comes in. --> Minimum amount available
Loan, minus rent and textbooks, divided by amount of weeks left until holidays --> Maximum amount available
I always tried to stay under the minimum amount so I had money left over for the holidays, but then it's ok if you go over it 'cause you're way under your maximum! ^^