If you recently logged into your Battle.Net account, you may have seen something like this:
There has not been any announcement related to this by Blizzard, but one can assume that this will be similar to the World of Warcraft Starter Edition that was released in July 2011 and the StarCraft II Starter Edition that followed. Similarly, this Diablo 3 Free version will most likely be limited by level and not by playtime.
The amount of content in this version will probably be the same as the beta (up to the Skeleton King). This has been confirmed by players who managed to get into the starter edition because of a bug on the Battle.Net servers. They claimed that it was very similar to the beta with all five classes being playable. The only difference being the occasional nag screen promoting you to upgrade to the full version.
Not all accounts have been credited with the Diablo 3 Free game, but you can be sure that all of them will on the 15th May.
Forum discussions regarding this topic can be found here and here.