@Yshania: Most of them must be returned in 4 weeks, unless someone else want them. Some popular books (those in reading lists for undergraduates) are short-loan books, so I have to return within 1 week.
The books I borrowed aren't so popular (therefore, there are plenty of room to be researched ), thus I think I can keep until due date. But, I have one book in Italian (!!), and another which could be in the reading list...
"Strength without wisdom falls by its own weight."
A word to the wise is sufficient Minerva (Semi-retired SYMer)