How long do ESC's typically last?

Most people never find out because they upgrade to a newer model before their old esc breaks. I have some that are 10 years old, but I haven't run them in 5 years.

You're much more likely to kill an ESC through misuse than for it to actually "wear out". I've got ESCs that are 15-odd years fitted in cars and they still work. There is nothing to really go wrong in a modern ESC - biggest risk is overloading it or getting it wet.

Assuming its a decent quality ESC in the first place of course.

life time of what the are in unless defective or abused..

the only thing I've found to have a "shelf life" are the electrolytic capacitors we've used across the power terminals (ex: Tekin power capacitor)..... These tend to get squirrely if they've been around for more that 10 yrs

My belief is field effect transistors can degrade during use. The time it takes to degrade is a mystery to me. This is why I buy new Esc every two years.

Capacitors would be the big concern, but they're almost always replaceable. Generally, everything has a lifespan though. I've got a keyboard that can handle 10 million presses on each key, but... that time will eventually come. The next thing would be the chips, as they age at even moderate heat levels (an average desktop CPU will handle over a million hours of uptime, even if only idling, but it will degrade). But I think you would literally have to run an ESC for hours a day over a decade before it would "wear out" the motherboard.

Mosfet just sitting there doesn’t wear out. Over stressing the mosfet will cause premature failure. Staying within limits the esc will last a long time. One way to see a mosfets age is on state resistance.

depends on the brand, most budget brands I've tested over the years tend to last about 6 months in off-road racing conditions, but they'll go as long as 2-3 years for on-road....
for light duty bashing with proper gearing a budget ESC can last 10+ years.

Generally my ESC's used in stock racing are pushing the limits with high temps and will wear out after about a year or so.

My ESC's used in mod classes will go for as long as I care for them, things that tend to shorten the life of an ESC are hard chassis slaps so it's very important to land the downside of a large jump rather than doing a hero launch to the moon if you want to ease the strain on your wallet.

I got a second hand Xerun 3.1 back in 2016. It's still going strong. I don't know what you guys are doing to wear out an ESC in an on-road environment.

It also depends on how you treat it.

Like in racing, at one event I was at there was a fast jump and you had to slam into the face of the next jump. Especially in 4wd mod buggy, you chassis slapped HARD. Thats hard on electronics. So if you do that kind of stuff a lot, expect things to fail sooner than if you didn't.

Other than that Esc's should easily last 2-3+ years. Don't hook them up backwards or run them outside of spec and quality ones last a long time.

They last right up to the point the magic smoke that powers then leaks out.

Can you be competitive in spec class with a 3+ yr old esc vs newer model(everything else being equal)?

For on-road, definitely.

I'd argue the only real 'updates' to ESCs in the last 3 years have been in the way one programs it (WiFi programmers & cellphones/tablets vs. plug-in programmers). While these make programming more convenient, they're certainly not necessary performance advantages. Further, since in stock classes we're not allowed to use many of the advanced programming features anyway, they're of even less benefit.

Plenty of fast guys using models released 4-5 years ago, even at the top level.