First, be aware that this project uses the Flexible Funding model. This is not like kickstarter; even if they don't reach their funding goal, any contributions you make still go to them. It's not an "all or nothing" deal like people are used to with kickstarter.
This campaign will receive all funds raised even if it does not reach its goal. Funding duration: August 03, 2013 - September 10, 2013 (11:59pm PT).
Second, there seems to be a bit of a contradiction on the timeline for this funding. The developers mention the following:
Our goal is to fund two to three man-years of full time work on Mailpile, with our first milestone in January 2014, when we will deliver an alpha version ...
Yet later they say (emphasis mine)
This is the Mailpile business model. As long as members of our community are willing to fund development (we will ask you to renew your membership in a years' time), we will dedicate ourselves to Mailpile and build the secure web-mail client you want.
Regardless of these inconsistencies, If they stick to the schedule then there should be a stable 1.0 release out during the first year of funding/development.
Following our alpha release, we will spend another 6-9 months fixing bugs, fleshing out features, responding to user feedback and getting the user interface translated to languages other than English. Our goal is to have a stable 1.0 release ready in the summer of 2014.