Cylinder low on compression

Low compression can cause misfire codes, damage to the converter due to unburned fuel in the exhaust stream and reduced power output.
Repairs depend on what is causing the low compression. A bad valve or valve seat or damage in the cylinder or leakage through a bad head gasket.
A leak down test would show you where the problem is and at least give you an idea of what needs repair.

https://www.2carpros.com/articles/the-reasons-for-low-compression

1 mile or 100,000 miles. It entirely depends on what the root cause of the loss is. A damaged valve could go for a long time or fail as soon as you start it.
I would do the tests and see where the issue is, then go from there as you can now decide on a plan of action. At the moment you are playing Russian Roulette.