This is an inverse relation between the lux value and the charge time; that is, the higher the lux value, the shorter the charge time.
Charge Time = k/(Lux Value)
Solve for k using the known point (50000 lux, 2 hours):
Charge Time*(Lux Value) = k
2 hours*50,000 = k
100,000 = k
Now solve for 806,000 lux:
Charge Time = k/(Lux Value)
Charge Time = (100,000)/(806,000)
Charge Time = 0.124 hours = 7.44 minutes
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You could also solve it using a proportion:
(x hours)/(2 hours) = (50,000 lux)/(806,000 lux)
Solve for x.