P0304 misfire DTC, and CEL

The check engine light in my 98 Outback has been on for a couple days, got the code checked and its P0304 cylinder 4 misfire. Car seems to be driving fine. I will replace a plugs and wires tonight, it has 120k miles and the original wires.

I was wondering when the CEL light will go out by itself assuming the problem is resolved? Does the fact that it has stayed on indicate a continuing problem, or does some time/milage have to elapse before it will clear itself?

Thanks