There is no answer to this question, and am pretty sure you won't find any documented studies in that regard, the ASIC mining industry is relatively new, and gears become obsolete quite too often, so they switch hands very fast from the ones who have high power rate to those who pay less until it eventually ends up in the hands of those who have free power or the gear could be scrapped.

With that in mind, most used gears have little to no information on them, you buy a used gear that has changed hands a couple of times until it got to the seller, you don't know if it has been mining for a month or a year, and thus whatever numbers you get to will be far from accurate.

The second issue is the fact that the quality of every mining gear is very different, every model is different, and even within the same model every batch will be different, you are talking about Chinese manufacturers that have little to no quality control, so if you ask me about Bitmain failure rate regarding the 17 series, It has no easily passed 80% failure rate in less than 2 years, if you ask phill he got lucky the s17 pro version and has almost no failure rate, we bought the same gears, probably a few months apart and have very different failure rate.

My S9i and S9j have been pretty solid, my S9ks and S9se died in no time, my M21s was been mining day and night with no issue, some other people have their M21s die in a few months, there are no official numbers on which you can count on, even the manufacturer themselves don't reveal those numbers, the only time Bitmain admitted a high failure rate was when Jihan Wu has to calm down the angry Chinese miners who were complaining about high failure rate on the 17 series, and he did admit that the rate was above 30% or more if I could remember the article correctly, but aside from that, there are no official statements what so ever.

As a general rule, you should assume that a brand new gear would last you 12 months, a used one will last half that period, while that isn't going to always be the case, it's not uncommon for gears to die in a few months, so if you place your business model on the assumption that you will get lucky and have those gears mine for years, you could and probably will be very disappointed.