Re: Basic marine deep cycle battery questions
Take the amp draw of the light and VHF, which should be written on the packaging somewhere, or stamped on the fitting. It will certainly be on the VHF on the data plate. Add those numbers together.
Then take the Amp hour rating of the battery, divide that by 2. Then divide that number by the total amp rating of the two devices. That will give you a best guess (in hours) of how long the battery will last. Actual number get more complicated since this ignores line losses, and in efficiencies, but will get you close.