I have a Bryant Unit which was installed in August of 1990 (Ultimate Plus 90) what is the typical useful life of this unit ?

Most furnaces have a typical life expedency of 16 to 24 yrs

And longer,, or shorter life, so much is according to how well it has been taken care of, kept clean, serviced on regular bases make a big difference

A freon leak that is pulled up into the flames of the furnace burners or pilot will shorten the life be many years.

Freon when burned creates phosgene gas and this is very corrosive to metal, especially the heat exchanger.

How can that happen?

Has happen more than you would think,, more so on down flow furnaces where the -coil is on the bottom below where the burners are