The amount of power available is directly proportional to the area of solar cells in use.
A 3 x A4ish fold up panel would typically be rated at something like 21 W, which rating would assume that it's sunny, the panel is aimed directly at the sun, and that the sun is fairly close to overhead.
21 W is equivalent to a current of 4 amps out of a USB port. A standard port is 1 A, and 2.4 A mains chargers are commonplace, so that's a fair amount of power.
However, when you look at the assumptions, you aren't going to be able to get that power unless you dedicate a lunch stop to charging, and that's of a limited duration.
If the panel isn't pointing directly at the sun you won't get full power - a 45 degree pointing error may mean you only get 1/4 of full power.
Light overcast will reduce the available power to about a quarter, or heavier cloud to an eighth. On the plus side, light from cloud is fairly uniform, so pointing errors matter less.
If it's within a couple of hours of sunset (or sunrise) the amount of power will be down to around a quarter.
A lot will depend on how well your luggage carrying arrangements match the requirements of the solar panel. Having all the component panels horizontal is best; it's not useful if they start to hang off the side or form a trough between two pannier tops. It's also good if they are far enough back that you don't cast your own shadow over them.