Well last update was Apple saying the mac mini was an important option in their lineup. So at least maybe there is still hope for an update based on that.
It was a pretty vague statement made by Apple management and they did not elaborate further.
We can consider several factors and guess what the most likely course of action would be.
The Mac mini is a low sales volume, low price computer; it is not a major part of Apple's revenue stream so there is unlikely that Apple will spend a lot of engineering resources on future development.
In order to improve margins, Apple is using low cost, low power notebook CPUs with integrated graphics with almost everything soldered to the motherboard including RAM and mass storage basically eliminating all avenues of user upgradeability. If they are to continue with the Mac mini (or something at its price point), it is pretty clear that cost considerations and simplified product development will be Apple's M.O.
Elsewhere in this Q&A forum, there is wishful thinking that there might be a Mac mini "Pro" or something modular or upgradeable like the old Power Mac G4 Cube. That does not seem to be a likely scenario based on the this product's sales volume, price point, and limited revenue generating potential. Note that the Cube sold at $1,800 in 2000; that's about $2,600 in today's dollars adjusted for inflation.
As for timing, that's anyone's guess. Apple has had several opportunities to refresh the current model with newer silicon, but has not acted.
At this point, it's all pure speculation.