Recording Degradation on Recording to reel to reel

I grew up with my dad's Ampex tape machine and hundreds of prerecorded tapes, jazz and classical. The tapes were all 7.5 ips. Back then turntables, tonearms and cartridges were not near the quality they are today and in spite of considerable tape his the Tapes were vastly superior to LPs. Today the situation has been reversed. Modern upper level turntable playback especially with 45 rpm records recorded from uncompressed files are vastly superior (and quieter) than those old tapes. I can not speak for 15 ips modern transfers as I have no experience with them. 

Back in the day, I had Sony, Teac and Revox reel to reel recorders and I bought many pre recorded tapes.  One of my favorites was Abraxas by Santana. When new, it sounded amazing!  But after more than a few plays, you could hear the hiss increase and as the hiss increased, the high end started to disappear and sadly, the tape started to sound like mud with tape hiss. 

Another thing to keep in mind is that those early 7" pre-recorded tapes were sooo close to the master, that I’m not sure one can get the same reproduction today, even with today’s one steps, and other pressings (SACD and 192 kHz files included). It’s one of the main reasons why they are so coveted even today. Granted, the S/N can’t approach today’s technology (and S/N is a definite priority with me), but the dynamics (on the right playback equipment) on some of these tapes is just mind-blowing. I only own 7 of them at the moment, but whenever I throw one on, and put on the headphones, I’m still to this day blown away by their dynamics, to the point I feel like I’m almost sitting at the mixing board. Don’t forget to keep those tape heads and capstans clean.

Pre recorded  tapes were duped at higher speeds.  Irrelevant to the question. In 40 years of intensive tape recording I could immediately tell if the best available tapes had ever been previously recorded upon. Cheap tapes were so bad to begin with it didn't matter.

Keep you heads, tape guides and capstan clean.

De magnetize heads and metal parts on tape path after some playing hours and keep the tapes stored properly.

Somewhere i have read that tapes can last 50 years, i think this claim is rather conservative.

To your question that would depend on many things, like tape quality, speed, tape machine, weather conditions...