I fully agree with all the above comments - you won't drain a good battery down with just a single light, LED or otherwise. Dual batteries provide great peace of mind and for anyone wanting to take it one step further, I used an automatic battery cutoff device called "priority start" on an old jeep which I usually left sitting for months at a time. When your voltage drops below 11.8 volts, a switch opens thereby stopping unnecessary drain.
Since this is somewhat on topic, an easy way to determine how long your battery will last is to add up the draw in amps of all devices you want to run at one time. These specs usually can be found on the internet. Then compare the total draw in amps to that of the amp hour capacity of your battery - that will give you an idea of how long your battery will last. The other way to do it of course is to purchase a meter which measures current draw and place the leads of that meter in between your power supply and the load - the meter will then tell you what the actual current draw is.
For instance, I have a kayak set up with two led lights and a fish finder. I power everything with a 12v / 1300mAh capacity battery. This means that my battery can provide 1.3 amps of current for one hour, or 15.6 watt/hours. My fish finder draws .25 amps and the lights draw a total of .12 amps - for a grand total of .37 amps, 370mA or 4.44 watts. Right off the bat I can expect my battery to provide more than one hour of run time because my load is less than my capacity per hour rating.
There are of course other factors (charge capacity, voltage threshold, temp, rate of draw, etc) which play into this and better ways to calculate this (watt-hour), but speaking very generally if I divide 1300 by 370 if I measure in amps or (15.6 by 4.44 in watts), I get 3.51, meaning I might get about 3.5 hours of run time out of my 12v drill battery. However, my LED lights turn off when voltage drops to 11 volts and my fish finder stops working at 10v. I'm not truly going to get a full 3.5 hours but its a starting point. Keep in mind you never want to fully discharge some types of batteries.
Lets say your anchor light has a 500 mA draw - meaning half of an amp. If you have a Group 27 marine deep cell battery, it likely has a 90AH @ 20 hour rate, meaning the battery can provide 4.5 amps for 20 hours. Your actual load is a fraction of the available capacity and you probably won't be out on the water for 20 hours- all this assuming your battery is in good shape.
