I have a carbon fork on a custom steel frame bike that I’ve owned for 15 years. I’ve probably ridden it 50,000 km, I’ve never crashed and the fork looks to be in perfect condition. It’s a Columbus fork. My question is generally speaking when should you replace a carbon fork? Would it be based on how much it’s been ridden, years, conditions, etc.

I thought I would ask in this sub/forum because bike mechanics who inspect frames and forks might have a good idea about such a thing.

I've heard of technicians doing a "tap test" on carbon fiber composite structures. Sit in a quiet place and very patiently tap it with a quarter every 1/2". Everything should sound the same, or gradual differences going into a joint.

The gold standard is ultrasonic inspection. It's probably cheaper to buy a new fork.

Never crashed. Looks to be in perfect condition.
I think you answered your own question.

Close visual inspection is your basic option. The "tap test" might add some peace of mind. My current CF fork (all CF) has 130,000 km on it. I weigh 80 kg. My previous CF fork (steel steerer tube) has 260,000 km on it. That bike is now on the rollers for the winters.

One Bike I have has a Bianchi Carbon fork Carbon steerer that's over 20 years old and around 30,000 miles.

For non mtb nor off road use, & it appears that there's no damage, it's likely a non-issue.

Thanks everyone! I wanted to play it safe and I think I’m good with this one a while longer

There is nothing that will intrinsically degrade on a carbon composite structure during normal use. The material doesn't have a fatigue life, and the resin on the surface of the structure should protect it from UV. Of course over time abrasion or crash damage because more likely, but in your case you're probably just fine.