Timer Variation works when "Timed Clips (vs Full Songs)" is set to ON. It is ON by default. (To change this setting, go to Settings.)
You can change the clip length by adjusting the slider on the Timer Variation page. If you make no other changes there, then each clip will play for the length set on that slider. While the maximum length is 60 seconds, research suggests that typical initial training should be around 15 seconds. (Beyond 60 seconds, either use whole songs or train with some variance, discussed below.) The minimum clip length allowed is 2 seconds; otherwise, there would not be enough time for a clip to viably act as a cause.
Early on in switch access training, there is some value in leaving clips set to one constant length of time; however, you run an increasing risk that the student might be learning to hit the switch after just that specific amount of time, rather than whenever the music stops. When you want the clip lengths to vary (plus or minus) within a certain amount of time, set that variance with the rolling picker; for example, you might set the clips to run for an average of 20 seconds (on the slider), plus or minus 10 seconds (on the rolling picker). That would mean that your clips would each run anywhere from 10 to 30 seconds (varying from clip to clip).
There is something that this app takes care of without your needing to worry about it, but you should still understand that it is happening. The app won't let you set the variance (on the picker) beyond the average length of the clip (on the slider); in other words, no matter how you try to set the variance, your choices are limited by the value on the slider. That way, you won't end up with clips trying to run for a negative amount of time. The app also imposes the 2-second minimum here. So, for example, if you were to set your clips to have a length of only 10 seconds (on the slider), then you would only be able to set a variance of up to 8 seconds (on the picker, i.e., 10 seconds minus the 2-second minimum). No matter how you set these values, the page displays a message with the results to help keep things clear, such as, "So, your settings will play music for 2 to 18 seconds before you hit the switch again (unless the song ends first)."
NOTE WELL: The new clip length will appear as soon as you hit the switch and the timer starts to count up, rather than before; otherwise, what happens is that the ending equation (i.e., "15 = 15" and similar) flashes for too short a time to represent the notion: "we have listended for the whole length of the clip, so hit the switch for to get a new clip with a new length." That works better for training than: "we already made a new clip without your having to do anything, so just hit the switch whenever you see a zero."