Calculating duration when changing playback speed
If you speed up, you simply devide the length by the factor.

For 1,5 : 1 / 1,5 = 0,666...

For 2,0 : 1 / 2,0 = 0,5

For 3,0 : 1 / 3,0 = 0,333...

It is similar to fast forward a video tape in good old times.

If you forward, you usually have factor 2,0, which means it takes 90 minutes to go through a 180 minute tape. If you fast forward, it is - let's say - factor 120,0, it can fast forward/rewind the tape in 90 seconds.