Linden Lab conducts previews when there are major features added or changes made to the client or server software. The preview is then used to allow residents to test the changes for themselves. This not only helps Linden Lab find bugs, but can also help residents prepare for features or changes that may affect them.
The Aditi Grid is a separate network of servers from the Agni Grid, and changes made here do not affect the main grid. Before a preview is opened to the public, a "snapshot" or backup of Agni is made, which is then transferred to the preview grid. Any changes on the main grid that a resident makes to their avatar or inventory after the snapshot date will not show up on the preview grid, and accounts created after that point are not able to participate on the preview grid.
No money or inventory can be transferred from Aditi to Agni. This separation is to allow for full testing of bugs without worry of altering anything permanently. It also provides a "closed" and "sanitary" environment in case any horrible bug should be found in the preview version. Often, extra money or land credits are given to the resident to facilitate testing of certain features.
The preview grid is often unavailable as new patches or internal tests are being performed.
Features or changes in preview versions are not guaranteed to make it to the final release. Sometimes a major bug is found, additional changes need to be made, or other things arise that can't be fixed in time for a timely final release.
Linden Lab usually tries to have previews last no longer than two weeks, but they often do. Previews should and usually do last until major bugs are fixed. And, typically, the release time of a preview can be said to be "when it's ready" or, more accurately, "when Linden Lab is comfortable with releasing it". So there really is no set length of time previews will take. This is a good thing, because keeping to a strict deadline of release for a preview could (and has in the past) resulted in major bugs being in the final release that cause problems for everyone. So, the longer the preview, the better the release will usually be. Also, the more people that are able to test a preview for bugs the better it usually will be as well. The shortest preview was for version 1.5 which lasted a week and resulted in 1.5 being regarded as the buggiest release to date.