All US states (I think) have some statute of limitations on debt collection, although each state is different. After this amount of time has passed (assuming that no court case has been filed), the courts can no longer be used to demand payment of a debt and the debt effectively ceases to exist legally. (Whether it ceases to exist morally depends on your personal moral system.)
But they will send collections after you long before that no? It happened to me (Scrips, CA), while we were trying to negotiate payment with the hospital and the bill was only $500 - I didn't want to pay it out of principal because we were told it was $150 flat rate at the time of service... they were very quick to pull the trigger.
Is it at all advisable to ignore collections and risk court appearance?
When the collections company called me I tried to negotiate a reasonable price with them but they wouldn't budge, I told them that I wouldn't pay and would just wait for seven years.
Next time they called I told them they had the wrong number and not to call again, and I never heard from them again, no letters, no calls, nothing.
Although I'm in Arizona, I don't know if laws are different in CA.