With the OEM exhaust installed, it now sounds a bit like those aftermarket exhaust intentionally meant to emit louder noise.

I do recall before the exhaust got louder I revved the engine in 1st gear close to redline. I always let my car warm up for at least 2 minutes, while the weather is cold.

It's nearing 170k miles.

 

I hope that was done with a fully warmed up car while moving...

You drive a (relatively) modern car, stop treating it like it was built with 60s technology. Start car, drive away. Keep engine load and RPM low until running temp is achieved. Anything else is not only a waste of fuel, but also contaminating the oil sooner due to additional fuel washing past the rings since it takes longer to warm the car without any load.

Now that we're through with that...

So you have an exhaust leak. Find it and fix it.

 

How long can I drive the car before it becomes a serious problem? I'm guessing I'll probably fail emission testing?

 

you either have a hole in your exhaust somewhere, or your muffler is defective.

 

How long you can drive it depends on how long you can stand the extra noise. Whether you pass emissions testing depends on where the leak is and how your state tests for emissions.

In IL, for example, long as the check engine light isnt on and the ECU says it is "ready" for testing, you'll pass.