how long do you have in maryland to file a lawsuit against someone that has not repaid a loan

It completely depends on the kind of loan at issue.  The general statute of limitations in Maryland for civil actions is 3 years, but the statute of limitations for promissory notes and financing statements can be as long as 12 years.   DISCLAIMER: This is not intended to be specific legal advice and should not be relied upon as such.  No attorney-client relationship is formed by this posting.