Charging a smartphone with lead acid battery
Assume your phone has a 3.5 Ah battery with a nominal voltage of 3.7 V, that gives you 12.95 Wh. Your lead acid battery may have a nominal voltage of 12 V and a 100 Ah capacity, that gives you 1200 Wh.

Watt Hours (Wh) = nominal voltage (V) * capacity (Ah) 

Assume the process of charging your phone is 100% efficient (it isn't), you get 92 charges from your lead acid battery. If one charge of your phone lasts 2 days that gives you 184 days from the car battery.

More likely is you'll get ~70-80% efficiency from the adapter, and a bit more inefficiency from the phone's internal charging circuitry, also, even deep cycle batteries won't be able to give you all their energy so you lose a little more.

The numbers are all made up, but hopefully gives enough information to work out what you need.