Capacitor.

[frank rabbets]

I notice the air bag ECU has capacitors. I guess other ECUs have capacitors. How long will they last?

 

[Towpack]

Anything from a few years to almost indefinitely depending on type, duty cycle, operating environment etc. Electrolytics are the most vulnerable to excess heat and humidity which can shorten service life considerably.
I don't think vehicle ECU's are that hard on capacitor life TBH.