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Average car starting battery is 40 to 60 amp hours when it is new depending on the size of the battery of course.
In theory you could charge a 40 amp hour battery in 4 hours with a 10 amp charger, but if you drain a 40 amp hour battery down 100% you've damaged the battery and you're never going to get it to hold 40 amp hours worth of energy again.
Older dumb battery chargers also only charge at their rated output when the battery is deeply discharged, as the battery reaches a fuller state of charge the energy going in tends to taper off, so a 10 amp charger might be putting 3 amps in to a battery when it's 80% full, this is of course good because absorption of the charge is needed and over charging causes the battery to boil and loose electrolyte (add distilled water) smart chargers can dump energy in to the battery faster and once they see the voltage top out they lower the amps going in and hold it at a set voltag, this boils the battery less and allows for a faster charge, it also requires a more expensive charger.
If you are charging over night tho, a 2amp or 5 amp charger that can float should work great.
Deep cycle batteries are not an ideal starting battery and starting batteries are not good with deep cycle loads, I don't see anything wrong with pairing a deep cycle battery up with a starting battery, the cable connecting the two will limit the energy from the deep cycle battery while starting and of course the load is split between the two so even under starting the starting battery will not be discharged as hard and the deep cycle will bring the starting battery back up a bit after the vehicle is running.
Other option that is even better and costs even more is a higher voltage deep cycle battery pack, a regular starting battery, and a dc/dc converter, this will allow the starting battery to be fully charged after starting, bringing it's voltage back up to 13.4v in the same way the alternator would, this will make your head lights brighter and put less stress on the rest of the electrical system.
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