If you have a decent voltmeter, I would suggest that you put in the time to do a daily test of the 12V battery for a month or two (for each car). Yeah, it's a PITA, but that will tell you what sort of drain is occurring on the vehicles' acc battery, and how often might be a good idea to charge (once a week, twice a month, every 2 months, ...) I did it for my vehicle over the course of about 10 days (and promptly 'lost it' - meaning my wife 'cleaned' ...). Personally, I would want to see how quickly (or 'if ever') the battery got down to 12.4V. I would *like* to keep the battery at above 12.45V, but wouldn't go apesh*t over it. (12.6V is basically 'full' - it varies a bit, but after 6 hours with minimal load, it's 12.6V.)
If the battery almost immediately (a day or 3) drops to 12.4V, that's something that would indicate more frequent charging would be useful. (At even 80% SoC a flooded lead-acid will start to sulfate under minor load - slow sulfation, and reversible with a full, saturation charge - but that's why one trickle charges.) If the battery is generally (most of the month) over 12.4V, charging once a month should be fine. And "12.4V" is just my opinion. It may be a bit overkill - but then I want the battery to last 8-12 years.
Usage patterns may affect how charged the 12V battery remains. The acc battery in the Spark EV is charged from the main battery when (a) actively charging the propulsion battery (not just plugged in) and (b) when in the 'drive' setting (and there's also a check done every few days but the voltage trigger, IMO, is too low). So, if you drive the vehicle 15 miles every 3 or 4 days, the voltage of your acc battery won't be 'maintained' by the Spark as well as if you drove 50 miles a day (charged while driving, charged while re-charging). IIRC, the voltage 'when charging or driving' provided to the 12V battery is 13.4V (it might have been 13.6V). Either is fine as a 'float' or 'maintenance' charge - but it won't refill a severely depleted battery, OR even saturate charge the acc battery. If you drive enough, the acc battery will remain fairly well charged (although not saturation charged). If you drive a little, not so much.
I don't recall my 12V battery dropping below 12.4V at any time during the 10-day check (full charge, then checked every night). But we usually drove the car 20-60 miles a day, every day. The thing to remember when checking voltage of the 12V battery is that 'wakening' the car (including simply opening the driver's door, or plugging something into the OBD port) will increase the draw on the acc battery, thus lowering the voltage you are measuring. Also, having the key fob 'too close' may do the same - due to the fob 'talking' to the Spark - so we started keeping the key 50 feet away in the kitchen instead of hanging on the wall in the garage.
So, if you are going to monitor voltage of the battery , try :
1- When you get home, park and pop the hood (so that you can open it later - I don't think that opening the HOOD wakes up any computers). Then walk away from (and lock) the car.
2- Go back 30+ minutes later to measure the battery voltage, so the draw has minimized, and the battery has been 'at rest' for at least 10 minutes (if not 25, or more).
I had a super, super, slow (but really intelligent) trickle charger : 0.8A. A 'pretty much full' battery would go quickly to 'mostly charged, now topping off' (maintain at 14.4V, dropping amperage until full), and then it would take 3-6 hours before it signalled 'maintain/float' mode (battery full, maintain at 13.6V). So I'd plug in late at night and it would be fully charged in the morning.