Unclear, and totally up to your DM, though you probably shouldn’t expect more than a handful of times a day. In real life (according to the National Institute of Health) venom can be harvested from snakes every 60 days, but that’s not entirely consistent with how much venom snakes seem capable of producing in game. As a DM, this is one of the tools you can use to make sure that players don’t harvest so much poison that it becomes problematic.