How long is it normal for an uncrowned root canal last for?

Depends on tooth: If the tooth is a molar, it is more likely to fracture from not having adequate protection with a crown. If it is a front tooth, often, it can have a root canal and survive for many years without a problem. It's all about the forces that are put on it. The more force on the root canaled tooth, the more chance of failure.

Unknown: I hope you mean that you have a permanent filling but no crown. A temporary filling will not last forever and a temporary filling will eventually dissolve and risk infection or tooth fracture. Molar teeth should have a crown, but front teeth may only need a permanent filling.