Usually get a flurry of bids early on, then it goes quiet in the middle, then towards the end, those with dusty phone/empty diaries get involved.
I think it's because of the way AW displays them to the SPs that you get that pattern, as they show "New" and "Expiring shortly" columns. And the search tools are not very intuitive, so in the middle you only get bids from those who really know how to work the RB system.
I've got one running that I started more than a month before, though I was planned to be away for two weeks of that period. So far I have a good number of high quality bids and expect more.
There is nothing to stop you emailing likely prospects with a "Would you like to bid on my RB?". Like any mailshot the response rate will not be high, but I've picked up a couple of very good bids that way.