On a good day, 15-20 mins, rarely less. It can be up to an hour, but that's usually when I've let her sleep a little too long during the day (she's soooo hard to wake, she sometimes ends up getting an extra 15 mins on her nap before I can persuade her to get up). She really does need time to process and wind down though, she will chat away to her friends about her day, then arrange them nicely in bed, sometimes shh pat them a bit... and finally lie down beside them herself. I did at one stage think she was taking too long and must be a bit UT, but I tried a later bedtime and she took just as long to settle and was tired the next day. So now, as others have said, I tend not to worry so long as I'm not getting call-backs (or not more than one reasonable one), and it's not getting tooo late.
Tonight she was out in 5 mins. That really doesn't happen very often!