I can't imagine planes literally being completely flat as they land, a significant safety margin is clearly required.
They won't be flat, but adding the margin
on bottom doesn't help with battery cycle life that much; most of the cycling damage is done charging near top, especially at high charge rates. To
have that safety margin, the cells would have to be charged near 100%; and given the hectic schedules, at significant charge rate. Basically it would need to be some 35% - 95% cycles in one hour which works out some 0.6C average. You are not really stress-quick-charging, but not doing easy cycles either, quite close to "standard" conditions actually which result in some 500-1000 cycles.
So while 500 cycles might not be the exact limit, 1000 might be achievable with the current tech, mzzj is mostly correct in this point. You can't derate the cells much to increase the cycle life. You can't slow down charging already at 80% like Tesla does.
But I do question the assumption of the nearly-50% jet engine efficiency used by mzzj. I suspect this number is at highly optimum conditions. Designed to provide good overall efficiency for long flights, jet engines are likely a lot more inefficient during taxiing, takeoff, climb, and again, when running (near-)idle during descent. Electric shines in providing good efficiency at large dynamic range, important at short trips.
In order to make reliable calculations, I think we shouldn't try to back-calculate flight energy usage from the fuel consumption times assumed efficiency, but instead go more directly at the source and use actual mechanical energy requirements, mostly regarding aerodynamics. But sadly I can't contribute with actual numbers here. I know it's so easy just to take the well-known fuel consumption numbers for existing passenger jets then work from there, but there are quite some many hidden assumptions.
In any case, I don't think we can twist the reality to make this viable with the
current battery technology, not even close, but OTOH, it's hard to predict what happens with battery technology.