Before you expect your book to die at all, go over to Dean Wesley Smith's blog and read the posts on long-tailed publishing. Ebooks can theoretically be in print forever, and even after the initial rush of sales, which may takes months to taper off, the long tail means that you can hope to still sell far into the future. Of course, if you've been following up with more new releases, each of those books will be doing the same thing.
I would expect about a month of those sales (30-40/day). After that your sales will either start to double, or they will drop off. For me, once I reached my genre's top 100, then my book went nuts, hitting the Amazon Top 100 and climbing. But, I am not expecting the sales surge to last for more than a month or so.
Everyone on this forum suggests writing another book ASAP. When is what I am doing.