There are many variations of WBTB. You need to experiment and find the amount of time that works for you. You should check out the following post if you haven’t already. Hopefully it will give you some ideas, but remember it is just one way to do it.
How to make sure your WBTB won’t fail
Personally I usually stay awake for around 15 minutes. However, some people stay up for 30 minutes or an hour while others only stay awake for a few minutes. Doing something related to LD’ing while you’re awake, such as reading/writing in your DJ or reading LD4ALL forums may help as well.