I have often heard seven years but have also heard people say that entries have been on longer.
I do know this for a fact,
when you owe a debt the entity may try to collect it for a while, maybe up to 60 days or so.
The entity may at that time sell the debt to another entity for a lessor amount than is owed. That entity will attempt to collect for some period of time as well and if unsuccessful sell it off to another debt collector.
Point is, each time its sold the new owner pops it on to your credit report, so who knows how long it could stay in the pipeline. I guess until the debt becomes so old its value becomes worthless.
Hope this helps