Try not to confuse voltage rise with a capacity increase. In order to get current flowing into a battery you need to increase the voltage. A voltage increase while charging is not representative of state of charge until you can hit and hold an absorption voltage for a while. If you are at absorption voltage (roughly 14.4V) you can safely guess that you're at close to 80% state of charge or higher. This is with healthy batteries. Batteries suffering from sulfation will come up to 14.4V well before they are actually at 80% SOC and this is then misleading. If you have an inexpensive shunting controller then the last 20% of charge will take a loooooong, loooong time regardless of sun.
A 30W panel will pump out about 1.8A for 3 - 4.5 hours per day, roughly. This is also with no shading. So if we figure 1.8 X 4 hours this = 7.2Ah per day of charging.
With a 100Ah battery at 50% state of charge 50 Ah's, plus about 15% charge inefficiency, would need to be replaced or 50Ah X 1.15 = 57.5Ah.
57.5Ah / 7.2Ah per day = 8 days to get a single 100Ah battery from half full to full, in perfect conditions with a 30W panel.
A 200 Ah bank would take approx 16 days to reach full, from half full...... These are averages and you may do slightly better and you very well can do considerably worse. A 30W panel simply can not take a group 24 battery from 20% state of charge to full in a couple of days..
It your OB motor or alternator can get the bank back to 80% state of charge regularly then base these figures on 20% of the capacity.
A 100Ah battery at 80% SOC - 20 Ah X 1.15 = 23 Ah / 7.2 = 3.2 days to go from 80% to 100% in perfect conditions.
Shunting controllers will get you back to 80% state of charge and do just fine for that but then they really slow down, due to their ON/OFF nature, and can drastically impede the time it takes to get from 80% - 100% state of charge. These controllers can add multiple days to the "topping up" portion of charging...