A day’s worth of cruising may very well not fully charge the batteries, depending on the size of the bank and the depth of discharge. It may do, but guessing isn’t the best way to look after the batteries. Only by monitoring them can you actually know how charged they are. Either by taking a hydrometer reading or by measuring tail current at 14.4V+.
As to how long you can can survive without hammering the batteries, have you done an energy audit? Without knowing how much you use daily (or nightly or whatever) you don’t know what percentage charge you are removing from the batteries per day. If you’re only removing say 20% of their capacity then you could go two days, but if you’re using 30% of their capacity then you really shouldn’t go more than a day if you want to maximise their life.
The answer to having a high daily usage isn’t simply to raise the battery capacity to cover it because you now have to consider how you’re going to replace that energy. A high depth of discharge simply won’t be replaced in the 12 hours between 8am and 8pm if you have a tiny alternator for instance.
The answer to good battery life life comes from monitoring them and following three steps:
1. Do an energy audit to estimate your daily usage.
2. Work out how you will replace ‘1’ daily (or 2 x ‘1’ every other day if you prefer). If you can’t balance this then reduce ‘1’.
3. Once 1 & 2 are balanced, buy a battery bank of at least three times ‘1’ if you want to last a couple of days between charging.
Oh, and with the current low price for solar panels it makes absolutely no sense these days to not have any - a decent solar installation can handle your entire daily power requirements in the summer.