I've been trying this out and, thanks to them giving me two weeks free rather than one month, crept over into the paying for it time.
Now, I started off on an 'unlimited' plan, in this case giving me one DVD 'out' at any one time, but with no limits on how many I have in a month. I switched to a cheaper 'four per month' plan on the 29th Feb because, in practice, that's how many I got: they'd send one, it arrived two days later, I watched it and sent it back same day (the joys of not having a day job), they'd acknowledge receipt a day or two later, say the next one would be sent "immediately" and then send it out the next day, typically after the usual last post time. Repeat. Throw in weekends, and you can work it out for yourself: one a week, basically.
But now I'm on a limited plan, for the first time ever the next one has been dispatched the same day as they got the previous one, and in the afternoon, not evening too. It may well arrive tomorrow.
I know I'm cynical, but doesn't that strike anyone else as a bit of a coincidence?