if you check the technical data of that exact device in the internet, it says it can produce a power of 220 mA at a voltage of 5.5V for the solarpanel.. (fits to the 1.2W claim on the package).. as the battery is 10,000 mAh it would take the solarpanel at least 45h to charge the battery to 100%... probably 30% more due to loss during charging.. so it would be around 60 sun hours... divided by 10 sun hours a day this would be 6 days for 100%.. this fits perfectly to the 2 days and 35% you measured.. if the 35% charging however would refer to your smartphone that only has a battery of 5.000 mAh this would be a pretty week result.. but possible if one considers further loss charging the one battery with the other and maybe missing a few sun hours each day..
but this is of course not a proof that solar power does not make sense.. this devices seems to be equipped with thin film micro solar cells... crystalline modules have double efficiency in comparison.. and the full space on top of the device can not be used to produce electricity as you need not one bigger solar cells but 10 micro cells to connect them serially to get to the required Voltage of 5.5 V... so usually you would get at least 4 W instead of 1.2 W out of that size of that device...
to sum it up: those devices only make sense as emergency or remote power device.. not for "saving money" replacing grid power instead...
If you put the big solar modules / panels on your roof top or small scale on your balcony this is a completely different story.. pay-back time economically and ecologically is fantastic..