I'm not exactly sure, but I haven't suffered a week of no-scans. I've had two sites start scans at the same time, then have just one get kicked off due to the high load.
If I manually try again periodically, I can luck out, but my sites don't go more than a couple of days without a scan.
I'm guessing that Wordfence's popularity has something to do with it. They've upped their user base by 25% in the past few months. Their attack rate is nearing 35k/sec pretty consistently, compared to the 20k/sec a few months ago. They need to increase their server capacity.