If a 100 watt solar panel put out 6 amp per hour in 6 hours of sun you would gain 36 amp with no use. How long would have to drive (on average) to get 36 amp back into your battery
In simplistic terms, my engine alternator is capable of putting out 160 amps, and my converter function is capable of putting out 100 amps. So replacing 36 amp-hrs would take less than 1/2 hour.
However, both of those can be throttled down based upon the existing battery charge voltage, and the charge rates the converter and alternator are built to deliver. For instance, the converter has three stages of charging ending in a trickle charge. The solar charger should modify it's charge rate as well, to not overcharge the batteries.