How long do we have to collect money from a client?

Statutes of limitation for breach of contract run from the time of the breach (or sometimes from the time when the breach was discovered.)  If the client breached by not paying you after you did the work in June, 2015 (as it it not yet August, you clearly haven't yet worked for the client in August, 2015; somewhere, there's a typo, but not sure where), you are well within any limitations period (unless your contract provides for a shorter one, which I doubt.)  I don't know what the Maryland statute of limitations for breach of contract is, but I have never seen one less than 2 years from breach.