How long do I have to validate debt with collection agency?

hi i live in pa and im being hassled by a collection agency over and old contract i defulted on with guardian protection home securty the sent me a letter in november 2005 and i called them and set up a repayment plan but they are posting negitive on my credit report i never stoped paying them there repayment plain so now im asking my self how did they come up with the dollar figure they say i owe them so is it to late to send them a verication of debt letter and is there a statute of limitations