What is the maximum playing time in a reel to reel tape recorder?

There are two things that define the playing time of a tape. The first is the length of the tape and the second is the speed at which the tape runs.

The largest reel commonly found is 10 1/2" diameter. Normally, 2500feet of tape can be held on a reel but thinner tapes allow a longer length of tape on the same size reel. The longest length normally found on 10/5" reels is 3600 feet. The thicker the tape, the more robust it is and the less "print through" will happen. Print through happens when the magnetic information on the tape copies itself to adjacent layers of tape. It can sometimes be heard as a faint echo either before or after the main signal is heard.

Tape speeds vary from 15/16th inch per second up to 30 inches per second. The faster the tape runs, the better the quality will be.

So, a 10.5" studio master tape running at 30 IPS will last a little over 16 minutes.

A thinner tape running at a very slow speed (15/16 IPS) can last for as long as 5 1/2 hours. This was a typical arrangement for radio station logging tapes used to record the output from the station.

If the arrangement of the tape heads allows it, the tape can be turned over and played in the opposite direction, so doubling the playing time.

Tape has been almost entirely replaced by digital media and obtaining open reel tape is becoming more difficult. It is hardly surprising: A hard disk is a similar cost to a good quality tape and recording capacity can be measured in weeks rather than minutes or hours.