According to my Electronics fella at work, who builds and maintains our batteries (film industry, we handle A LOT every day) the "ideal" scenario to charging batteries is in multiples of 4.
So, although not set in stone, a 1amp charger would be "ideal" for a 4ah battery, or 3amp charger for a 12ah battery, and so on.
Obviously a 1amp charger with a 12ah battery does work absolutely fine, but will take 12hrs from flat in theory, if maths are to be believed, although you will never flatten a lead acid battery down to zero during usage, they don't work the same as Lithium batteries which will use up every drop of power. Low ampage chargers with big batteries are not really a viable option in our game due to not having time on our side while out filming.
For the record, when needed (once, maybe twice a year) i use a 4amp charger at work, and it normally charges my battery to full in less than an hour.
The chargers will boost them up to just past 80% then it will turn into a much lower trickle charge for the remainder, switching itself off once full, as i would presume all modern chargers should do these days.