Re: "used drones to observe the Ukrainians' movements"
> Starting from today, how many days would it take the Russian Army to reach the Channel and be prepared to cross?
It took the Wehrmacht 15 days from the beginning of the offensive against Belgium, France and the Netherlands until they had the BEF with its back to the sea at Dunkirk.  Of course, the Russians might have to cross Poland and Germany too, or maybe they'd take Ireland first with an amphibious landing? Granted, the reverse-D-Day operation would take much longer to mount, but not as much as you might think, if the situation got to that point.
In any event, one wouldn't have time to train conscript troops to the standard required for modern land warfare, and possibly not enough time to mobilise anything more than a short-notice regularly re-trained reserve. 
Operation Seelowe (Sealion), the German plan for invasion, was not mounted because the Germans couldn't achieve air supremacy over the English Channel. But it was touch and go...
 You've seen the opening credits for Dad's Army, right?
 As far as I know. I Am Not A Strategic Military Planner!