I don't bother with istock any more. The commission cut was the final straw for me. I often had upload problems and they sold a lot less than Pond5. Reviews usually took over a month for me.
I keep all my clips below 20 seconds, it makes the files smaller to upload, they are accepted everywhere and I also wonder if buyers prefer shorter clips? I suppose it depends on the subject and it gives buyers room to edit if it's longer but they also have to spend more time downloading a big file that they might only want a few seconds of.