Most men have only about a 10-year period when they care about their semen quality, you can use the sauna the other 50 years.
Other 50 years? What happened to the other 40 years?
I assume ~80 year lifespan and that you start going to the sauna as an adult.
Why would you only go to the sauna as an adult?
In any case, how long does sauna's supposed deleterious effect last? Probably not more than a few days?
For sperm metrics to return to normal, a few months.
I would be very surprised if it was above one day.
Don't sauna in the few days before you plan to conceive. Bikes, among other things, can also screw up your semen. It's not that uncommon, and it's only temporary