I don't really understand the question. It's not like a complete action with a start and a finish. If I'm not thinking about it, I often start doing it. When I realize I'm doing it, I don't usually know how long I've been doing it. A big part of my social skill training has been learning to pay attention and try to notice when I'm doing it as quickly as possible, since it looks very strange and puts people off, but even after years of trying, it's hard to always be conscious of it.
"It" can be anything from rocking back and forth to blinking rapidly, running my thumb over each of my fingers in sequence, making noises with my mouth, bouncing up and down, or any number of other things. It happens most often when I'm nervous or stressed, but can happen even when I'm calm and relaxed. If I'm really stressed, sometimes I will consciously start stimming without trying to stop it because I know it might help me calm down.
Something you do at a specific time sounds more like a ritual than a stim.