"I was just thinking if you could carry some sort of portable solar sheet, how long would it take to charge the batteries for another 10 minute flight? "
That's readily calculable for a rainy day since there's a near-zero in the terms.
Otherwise, more info is needed; possible variables:
Solar panel output under assumable lighting conditions.
Assume 80% charging efficiency, maybe less.
Need capacity of battery. No, you don't My error, I got "a full charge" notion stuck in my head.
Need amount of energy needed to go (wherever) in ten minutes. This would be approximately ((ten minutes ÷ time in minutes to totally discharge the battery at flying output rate) x battery capacity.)
I'm sure there are others I'm missing.
But, for that rainy day, assume at least 24 hours + charging time as calculated in the above mish-mosh.
:)
Terry