This is an interesting topic and I've been studying it a lot lately. I've read of knitters who knit 60-80 stitches per minute. I've also found some great articles on Pinterest and YouTube that discuss how to knit faster and show you different styles.
I found a couple videos of the yarn harlot showing 2 different ways she knits. She is extremely fast. If you'd like to see some of the article and videos I've found check out my Pinterest page under the topic "fast knitting" - my username is flirtyknitter there. To connect with me on Twitter, you can use that same name.
I'm working on speed with continental knitting but I don't have a great time per minute there. I'm using a flicking method that I found on a YouTube video and I've been working on mastering a quick way to knit using this method. It's similar to the one yarn harlot uses. I would be happy to look up the video for you if you're interested. Just shoot me a message on Twitter and I'll send you the link.
I've been timing myself using a variety of styles based on that video and I'm pretty proud of how much I've improved!
I would say I can knit now anywhere from 20 to 40 stitches in garter stitch. I read about the average knitters speed being 20 so I think I'm making progress!
Side note: I think I could be A LOT FASTER but I'm using a size 15 circular needle (plastic not metal - which to me is slower) and I'm using double stranded yarn. (I make a lot of lapghans and typically use the same pattern).
Anyway, I hope that helps someone or answers your question! The only concrete answer I found in this forum was 60 stitches per minute which is amazing! Gold star!
Find me on Twitter and let's connect! I look forward to meeting new knitters (and other needleworkers!!)
Flirtyknitter on Twitter