Found this at Optima's site regarding battery runtime and "amp hours":
Amp Hours
The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.
Reserve Minutes
Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)
The D34/78 is rated for 55 amp hours and 120 reserve minutes, so one battery will yield 2.75A steadily for 20 hours (55 amp hours divided by 20) before reaching 10.5V and being classified as "fully discharged." It will carry a 25 amp load for two hours before needing to be recharged.