Don't go by mileage, go by stretch. 12 links should be 12". When that is 12-1/16", you should replace the chain. When it hits 12-1/8" you should replace the entire drivetrain as well since it will have worn to accommodate the increased pitch.

1/8" BMX or track chains tend to be stronger due to their generally overbuilt nature and will stretch less. In particular, KMC's Z chains are tough as nails but also heavy and tend to be noisy. If you don't have 1/8" cog and chainring, they will probably be extra noisy as they can slop around on the gears.

Keeping your chain clean and properly lubed will lengthen the lifespan.

Both KMC & Wipperman (Sachs, I believe) have nice 1/8" and 3/32" single-speed/BMX specific offerings (unramped plates). In my experience, the heavy duty HX chains actually run quieter. For sure get a corrosion resistant (nickel plated or stainless steel) chain. It's totally worth the small premium to never worry about a rusty chain. I've got a 1/8" Z chain with thousands of miles and no measurable stretch.