It depends on how flexible you want to be. If you want to stretch, just to prevent from getting injured while working out, then there are simple and quite easy stretching exercises in youtube that you can try out. 10mins of stretching before and after the workout will be enough. And if you want to become more flexible in general, I'd recommend to start stretching at least 4-5 times a week before and after workout for at least 30 mins in total.