Usually 20-30 minutes during a nap; sometimes longer if I'm not relaxed.

Now, as for trying to get into SP in the middle of the night... whew, it took a good 40 minutes or so in the (very) few instances I tried it at that time. But that's okay - I'm not the kind of person who enjoys getting hallucinations and weird sensations in the dark, I prefer entering SP during the day.