Estimate audio recording length.
Audio duration follows directly from file size and bitrate, because bitrate is by definition the bits consumed per second of playback. Convert the file size to kilobits — megabytes times eight times 1024 — and divide by the bitrate in kilobits per second. A 48 MB file at 128 kbps therefore runs about 51 minutes. Variable bitrate encoding complicates this slightly: the quoted figure is a target average and real files usually land a few percent below it, so the duration comes out a little longer than the nominal calculation suggests. Channel count does not change duration at a given bitrate, only how that bitrate is shared.
Duration
Seconds = file size MB x 8 x 1024 / bitrate kbps
Size per minute
MB per minute = bitrate kbps x 60 / (8 x 1024)
Bitrate is the data spent per second of audio, so for a fixed file size it directly trades against length. Halving the bitrate doubles the duration you can fit into the same file — which is exactly why spoken-word podcasts are encoded at 64 to 96 kbps while music uses 192 or higher.
Not quite. Mono at the same bitrate produces the same file size and duration; what changes is that the whole bitrate serves one channel, so quality improves. To actually halve the size you encode mono at half the bitrate, which for speech is usually indistinguishable.