You want to stay awake long enough to be sure your brain is activated and focused on lucid dreaming, but not long enough to make it difficult to fall back asleep.

How are your WILDs failing? If you are falling asleep too quickly, then try to extend the time awake, and maybe read more or do some math problems. If you are lying awake for an hour or two after returning to bed, you need to reduce the amount of time you are up and about. Some people stay up for an hour, and some are only up long enough to jot down a few notes in their dream journal.