I'm good with one post a week myself. You already know of my own depression issues, GM, but only [i]extremely[/i] rarely have I dropped things where I've already had an effect on other PCs - and only for good reason. I can take an RP being really slow, but usually not much faster than one post a week, for the moment at least.
I don't need much incentive at all really to stick to something longterm, as those are my favorite type of rps. Just all the extra time for character interaction is cool.