They could conceivably go about this until the statute of limitations runs out, but I wouldn't expect them to. For information on how to answer a complaint for debt collection, go to the link below. If you dispute the debt or are unable to pay it, you should meet with an attorney.
If by "send" you mean that the summons is being delivered through the postal mail, service may not be proper as the service of most summons must be done in person, not through the mail.
Debt collectors must abide by specific regulations. You can check whether this debt collector is in compliance of the law. Noncompliance exposes the debt collector to being sued by you.
You should review your specific facts with your attorney to find out your legal options.