We expect to reach beta in around a couple of months, but in general you should know that we've worked hard not to break API compatibility between releases, even during the alpha phase. So far there's only been one release which modified documented API functionality, and this was explained in detail in the README. We certainly expect no incompatible API changes between now and the 1.0 release.
We expect the beta phase will last around 6 months, while we expand our internal test suite, continue performing security reviews, and allow any hidden bugs to be found by the user base. So that means a "1.0 release" late in 2017. However MultiChain is already being used in some production systems, and it's pretty solid as an alpha goes. You are of course also free to review the source code which has just been made available at Github.