Tesla now has batteries to the point where you can recoup energy for a 4-hour drive in 30 minutes. That's in full-on production, you can buy it and do it at retail today. The biggest challenge Tesla has is keeping the batteries cool during this slam charging; and in an airplane we have a huge surface area being blown by a slipstream. Have the battery pack be a thin layer bonded to the underside of the wing, and you got that licked!
So I think we can safely say woth emerging tech, you could recoup energy for 40 minutes of flying in 5 minutes of tanking.
The problem is, this defeats the entire purpose.
You are spending between 4 and 10 times the fossil fuel to get the tanker airborne, on station and generating the recharge power, than if you just ran the e-plane on a gas engine in the first place. And wasting a whole second crew, and having to build the second airplane with positively ginormous auxiliary generators... The whole thing beomes an exercise in futility.
Use fuel for that mission. Offset fuel burn elsewhere.
So you want an e-plane? OK, build a gas plane and buy the Postal Service ten electric jeeps. Not only will it work better, with proven COTS tech, it'll be cheaper to boot.
The realistic picture for green aviation is to beat the Cessna 182 into feeder wires for high speed rail. Let the electric truck deliver the epoxy, rolls of carbon fiber cloth, electric motors and battery modules. Then, build something totally new, with new design factors from prop(s) to rudder.
Honestly, given the numbers Tesla is proving in production, I am optimistic about electrifying all sorts of things previously thought to be un-electrify-able. I see 1200 mile flights with an electric plane to be perfectly feasible: fly for 4 hours, land, supercharge while you get breakfast, fly for 4 more hours, lunch, 4 more hours and home.