I don't know about precise percentages but yes, the "no response means no" thing, whether stated as policy or not, is becoming more and more common. I think enough agents have been burned by nasty responses to rejections that they're finding it easier (and probably less exhausting) to not respond. And response times can vary wildly. When I was last querying agents a couple of years ago, I never expected to hear anything sooner than 3-4 weeks, and usually assumed after 8-10 that it was a no...but sometimes I'd get requests for mss. at 12 and beyond.

It's a messy business and not really amenable to quantifying, unfortunately.