I don't expect an update to the 12.9 inch one until 2017 at the earliest, and I've heard no rumors about the next version. Though there's a possibility of a revision in November, a year after the first version launched. It would be waiting on at least a new processor, likely to be called the A10 or A10X. I personally wouldn't bet on November though.

Apple processor precedent mostly has been iPhone first, then a possible X variant for the iPads. The 6s shipped with the A9 in September 2015, then the iPad Pro got the A9X in November 2015 and March 2016.

Prior to that though, the A8 came out in the iPhone 6, and iPad Mini 4 in September 2014. The A8X then went into the Air 2 a month later in October 2014.