This maybe a pretty simple question, but I can't get around it for some reason. When I speed up a video to say 1.5x or 2x, how do I get the total time that video would take?
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.