It depends on the battery -- how many amp/hours is it rated at? It will say right on the top. There are group 24, group 27, group 31, and so on -- the capacity goes up and up. A group 31 (which is what I have) is rated at 110 amp/hours, which lasts us about three days of dry camping. (I have two of them, and I swap them out after three days.)
A simple multimeter, very inexpensive, is a great tool -- you can monitor the voltage in your battery. When fully charged, it reads 12.7. When it gets down to 12.1, you are at 50% state of charge, and it is time to re-charge the battery -- otherwise you could damage the battery and shorten its life. Please don't ask me how I learned this painful lesson.
You are doing exactly the right thing -- a short dry camping trip to see how it goes. Once you get a handle on your consumption patterns, you can try more adventurous trips.
Be sure to ask questions -- we were all "boondocking newbies" at one time, and we are happy to share our hard-earned tips on how not to make the misteaks we made!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)
Our trips -- pix and textAbout our trailer"A journey of a thousand miles begins with a single list."