Welcome!
Engine wear is 100% based on how it's used.
If the previous owner raced desert or MX for 90 hours, then yes, I would expect a top and bottom to be due or done.
If it was ridden on trails slowly with young kids or group riding, I would expect 100 hours for a top end and 200+ for the bottom.
If you read the manual it probably recommends a top end every 10-15 hours of MX use. I would be rebuilding before it was needed, if 'calculated flight' was my primary use for the bike. (MX)
The valves need to be properly adjusted as well, or they will wear faster than normal.
My answer is that regular valve spec, leakdown, ring end gap, and crank clearance readings through the life of a top/bottom end gives the best indication of life expectancy of any given build. You can develop a curve based on change over time to anticipate end of usable life for each of these components.
-BIG DAN