The amount of time that a creditor has to legally collect a debt from you can vary from state to state and the time they can legally collect and chase you for a debt is from 4 to 7 years but in most locations it's 7 years.
That means that the debt collection agency can only legally chase you for the debt for up to 7 years and after that 7 years they cannot force you to pay back the debt or take you to court and sue you for the debt after the 7 years have passed.
The 7 years is the statute of limitations for a debt collector to chase your for the debt.
After 7 years any collections accounts placed on your credit report should also be removed from your credit report.