It’s pretty normal for all keepers to experience loss of livestock and even missing fish entirely.
When all is established I would expect most keepers to keep fish in optimum conditions for at least 4-7 years, dependant of fish species but more than this is normal too.
I had CPDs, Lambchop rasboras and endlers in decent numbers in an established and all lived for at least 4 years, but most lived for approx 6 years, the endlers were the most long lived fish I had, got them as adults and had them for at least 7 years, the last endler died of presumably old age or natural causes possibly as was perfectly fine the day before, was sad to lose that one tbh.
The longer you long lived fish, the more attached you get to them, well for me anyways!
Before I had those, I had Threadfin Rainbowfish and these are fairly skittish fish and most of these lived for between 3-4 years, I was a fairly inexperienced keeper at that time.
So it all varies between tanks set ups, tank mates, fish species and so on and so forth so there is no real set rules or guidelines for how long one would expect to keep fish alive for.