As a general rule, you may attempt to collect upon a delinquent account at any time. That being said, there are a number of considerations that may affect what you decide to do in any particular situation. In the specific situation described:

1)    You should probably consider the amount involved and whether you have an ongoing relationship with the patient.  If it is a small amount or if there is an otherwise acceptable relationship, you may make a business decision to write off the overdue amount given the time elapsed.

2)    If it is a larger amount and/or if there is no ongoing patient relationship, you might decide to go after collection of the account.  In that case, you can either assign it directly to collections or send a notice (since it has been several years since the treatment) reminding the patient of the amount outstanding.  If you send the notice, you might want to include language indicating you will assign it for collection if the amount is not paid within some number of days (generally 30 or 45).

3)    If the account is more than six (6) years old, it may be legally uncollectible.  The provision of services in exchange for payment is treated as a contract for legal purposes and the statute of limitations for enforcement of contract rights (your right to payment) is six (6) years.

As a general matter, you may wish to establish an office policy for billing and collections.  Once you send a statement, it can specify that payment is required within any specified number of days (there is no legal requirement here as to how long, just so the number of days is not different from something else you may have told the patient) – 30 or 45 days are fairly normal periods.  If not paid in that time, you could immediately turn the account over to collections, although a far more common practice is to send a reminder notice saying they have only 30 more days before it goes to collections.  It is not uncommon to then use a "final notice" saying that if payment is not received in some lesser number of days (normally 10) the account will go to collections.

You should also be aware that, while there are no set time limits, the process of debt collection itself is subject to the Fair Debt Collection Practices Act.  While this statute normally applies only to “debt collectors,” several court cases have applied it to “creditors” who were engaged in deceptive practices. Further, debtors have attempted to hold creditors liable for the actions of a third-party debt collector employing various common law theories (claiming that the creditor is liable for the actions of its agent, the debt collector), state consumer laws (which sometimes offer enhanced debtor protection) and negligent referral (claiming the account was negligently or improperly referred to the debt collector).  As a general rule these lawsuits have been met with limited success, with only the extreme cases making any headway in court.  Wisconsin has its own debt collection statute (Chapter 427 of the Wisconsin Consumer Act) which applies to both debt collectors and creditors.  Consequently if you decide to handle delinquent account collections yourself, you should review and become familiar with the State law.