The time it will take to Re-veg it there is no point re-vegging flowering plants/clones can take up to 3 4 weeks
The point to doing it is to preserve genetics from a plant that's in flowering.
Of course its slower to get back to vegetative growth with a clone from a flowering plant compared to a clone taken from one already in vegetative growth.
But getting clones this way is still WAY faster than waiting for the flowering plant to finish flowering, THEN re-vegging it, THEN taking the clones!
In terms of re-vegging flowering plants, what happens is that every bud site turns into its own branch. Yes, it definitely can be done, but if you want to re-flower the same plant again, you have to be very careful with pruning, otherwise you'll end up with a plant with a hundred small "popcorn" buds, instead of a few big colas.