The amount of time required to charge a given battery depends on its capacity in Amp-hours (Ah) and its nominal voltage. For example, the battery in my laptop might be 3.4v with 1.4 Ah capacity. If it has 50% charge then it will need 700 mAh to bring it to a full charge. If the nominal voltage of the solar collector is 6.8v then I will need a transformer to lower the voltage to 3.4v, and which will give me almost double the available amperage (minus inductance and resistance losses). As I mentioned earlier, the available amperage depends on the intensity of the sunlight and the size of the collector, which is pretty widely variable.
@woodcutter You’d be best advised to take both a spare battery and a solar charger, given how long it might take to recharge (in case of overcast, solar eclipse, or what have you.)