How long from "almost crawling" to "crawling"?

[BusyMissIzzy]

I suspect the answer to this might be "as long as a piece of string", but I'm interested in others' experiences.
DD is desperate to crawl, and recently it seems like she could be nearly there. Sometimes she just flails her arms and legs and gets frustrated, but now she's started to get up onto hands and knees, but just doesn't know what to do to actually move. She does sort of manage to move around somehow (in that she's never quite in the same place I left her!).
So, how long did it take your DCs to master crawling, once it seemed like they were "nearly" there?
I should add that she's only 5mo, so I'm in no hurry for her to crawl. In fact I'm sort of dreading it, she'll be into everything.

My son never has actually crawled, despite getting into position. He bum shuffles, he walks on hands and feet. And in about another week? He'll be walking.

So the answer is...how long is a piece of string?

definitely how long is a piece of string, I'm afraid! DD2 has been getting frustrated at not being able to propel herself towards toys etc for a good month now and is 6 months. I don't anticipate her crawling for a good couple of months though because she isn't strong enough to hold her weight on her arms/legs yet. DD1 was the same and crawled at 8 months....for a couple of months before she finally crawled she would get to where she wanted to be by rolling around the room.