A year is about right, with perhaps (probably?) a diminishing ROI afterwards. I'm running a similar setup, but with two USB miners plugged into a powered hub. I get right around 670-671 Mhs, but I'd love to be able to get up to 1 Ghs. I don't know why, it's not like it'll ever be profitable at this low level. A BFL Jalapeño would be great, but who wants to wait months and months and months?