In a single month of being with Shutterstock, I have managed to generate 200 downloads, and almost enough for a payout. Almost.
Other folks I've talked with in the forums there have said it's taken them months to get to the magic amount for a payout, and I'm ***almost*** there! If you factor in last month's downloads, generated in only 8 days live and online there, it's not a bad start.
ShutterStock uses a subscription model, a buyer gets "X" number of downloads for a flat fee. Two other agencies I sell through are starting the same business model. iSTock and 123RF thus far.