The release candidate build that is currently distributed to both Technet and MSDN subscribers will not expire before June 1, 2010. It is very likely that the public release candidate on May 5 will come with the same expiration date. This essentially means that users who download the release candidate will be able to use it for free for the period of 13 months. The reason behind this rather large period in which the release candidate can be tested is unknown at this point. The Windows Vista release candidate expired on May 31, 2007 which meant an effective testing period of less than nine months.