Tape Durations - Reel to Reel
Tape Durations can be quite difficult to predict if you can't remember much about the content! Each step-change of recording speed will actually double or halve the duration. A four track tape can for example be twice as long as a two-track tape.

Some tapes are recorded on one side only, some on both sides. A standard 7" spool of stereo 2-track material recorded at 15 IPS might contain as little as 15 minutes of audio, but the very same tape recorded across four mono tracks at 1 7/8 IPS might contain over 8 hours of audio! This makes it jolly difficult to guess in advance quite how much is on the tape.

Recording/Running Speed - Reel to Reel
As a rule, the faster the speed the better the sound quality.

In addition to faithfully recording higher frequencies and increasing the magnetic signal strength and hence the signal to noise ratio, higher tape speeds spread the signal longitudinally over more tape area, reducing the effects of defects in or damage to the medium. Slower speeds tend to conserve tape and is more useful in applications where sound quality is not critical.

• 15/16ths of an inch per second (in/s) or 2.38 cm/s — is used for long duration recordings (e.g. recording a radio station's entire output in case of complaints, aka "logging")

• 1 1/4 in/s or 4.76 cm/s — This is usually the slowest domestic speed, best for long duration speech recordings

• 3¾ in/s or 9.52 cm/s — This is a common domestic speed and is used on most single-speed domestic machines

• 7½ in/s or 19.05 cm/s — This is generally the highest domestic speed and used by many radio stations for "dubs", copies of commercial announcements

• 15 in/s or 38.1 cm/s — These speeds were used for professional music recording and radio programming

• 30 in/s or 76.2 cm/s — These were used where the best possible treble response is demanded, e.g., many classical music recordings