A C60 audio cassette holds 30 minutes per side, and the tape is ~85 meters long (https://en.wikipedia.org/wiki/Audio_tape_specifications#Comp...).

The diameter of the actual disc in a 3.5-inch floppy is 85mm (https://en.wikipedia.org/wiki/History_of_the_floppy_disk#.E2...), so the circumference of the outer track should be about pi * 0.085m or 0.25m.

There are normally 80 tracks (https://en.wikipedia.org/wiki/List_of_floppy_disk_formats#Ph...), so if all tracks were as big as the outer one, that would give you about 20 meters.

In reality, inner tracks are shorter. I can't easily find the inner diameter, but from pictures it looks like it's about 1/3 the outer diameter. So the track lengths (circumference) should grow linearly from 1/3 to 100% of the outer diameter, which means the total track length should be 2/3 of 20 meters or ~13 meters.

To sum all that up, with 80 tracks, there seems to be about 13 meters of linear space the head can travel across, or about 15% of the length an audio cassette uses for 30 minutes. If you recorded at the same linear speed as a cassette, you'd get 15% of 30 minutes.

But running a cassette at half speed is definitely practical (https://en.wikipedia.org/wiki/Audio_tape_specifications#Tape...), and machines were even sold with a setting for that. So double that to 30%.

And there are a handful of floppy formats that use more than 80 tracks (one even uses 240), so let's assume 160 tracks is possible. So double again to 60% of 30 minutes.

And that's just one side of the floppy, so using both sides means you can double again to get 120% of 30 minutes. So yeah, it seems possible! (Which incidentally is NOT the answer I was expecting.)

If the 160 tracks thing doesn't work out, you could probably compensate slowing the rotation more.

On a side note, the two-sided thing creates the potential problem of a gap at the 15-minute mark. Apparently there are read/write heads on both sides (https://computer.howstuffworks.com/floppy-disk-drive2.htm), so I guess you could do the transition by recording a few seconds of audio on both sides, then cross-fading between them. In other words, start at the inner edge of the disk, reading from one side for 15 minutes as you slowly move the head to the outer edge of the disk, then cross-fade to the other head and continue back for another 15 minutes back toward the inner edge of the disk. You would need to have a good method for being sure you make this transition at the right time, though.

Or you can always just record 15 minutes of stereo sound, reading each channel from one side of the floppy.