Didn't take too long and as has been said DH reckons it was about 15 minutes, I thought about 10 but can't really remember! It's quite a pleasant little diversion and there was no queue when we rode, but I don't think it is worth a long wait.
I love going on the skytower and trying to spot all the sights from up there it's so cool and I also think worth the extra charge although I wouldn't Q for more than 10 minutes for it.