Historically they've been on an 8 month cycle of minor and major refreshes. It's been around 4 months since the major (Haswell) refresh. Figure June or July for the next spec bump.
The three major technologies to look for are Broadwell CPUs, nVidia Maxwell GPUs (unless they decide to switch back to AMD for this generation, which is possible but unlikely) and DDR4.
Broadwell will not be ready until at least Q3 2014 according to rumors.
DDR4 is going to be supported starting with the Haswell-E CPUs, meaning it could make it into next-gen. Apple was a very early adopter of DDR3 and might do the same with DDR4.
Maxwell mobile chips will likely not be ready by June.
Conclusion: the next MBP to carry all 3 of these advancements will be the next major refresh, available roughly one year from now.