Without much considerations and only technical, (ie. sort of a XXXD but mirrorless), I think pretty fast (they probably already have prototypes in the closet).
With its leverage of PnS, Sensor and DSLR knowhow, Canon can certainly enter the market very fast.
The question is more of eventual direction.
Do they want just a mirrorless XXXD series using existing DSLR lenses, or something more.
The Canon and Nikon lenses are typically not small, esp the VR, USM types.
The form factor will become rather odd (small body; large lenses).
A mirrorless system the size of existing DSLR bodies defeats the purpose of a mirrorless system or at least takes away one point that make a MLIC camera different from a DSLR system.
Then of course there is flange distance considerations. Canon is not too bad here, as its current DSLR flange distance is not too long and already it can take many other brand lenses w/ an adapter. Then again, that is a 'bad' thing for the comapany, as they may end up cannibalizing their own lens sales.
If its going to be new mount and new lens, then its a slower path to market.
In the end, its anyone's guess.
I guess we just have to wait and see.