This depends on how deeply you want to dive in with the process, and yourself.
A large part of the benefit of writing practice is that you’re making yourself a priority – putting yourself at the top of your to-do list – and you want to do that don’t you? I mean, this is it; this is life – the one you get* … so finding some time every day to sit with/work on the prompts is ideal. 20 minutes is great. More? Double bonus.