This somewhat echoes what Tom said, but I know when I query the group, I like to get plenty of feedback, so here is mine.
Right now you have asked a question that is somewhat impossible to answer accurately. It would be like either of these questions:
1) How much food can we eat each day and still not use it all up?
But..... we don't know how much food you are bringing along, how many people you have, how much they will eat, etc. So the answer could be just about anything.
2) How much can I drive each day and not run out of gas?
But... we don't know the vehicle/mileage, how hard/far you plan to drive, the size of the gas tank, or how much fuel you have in it.
The short answer for me would be that if you don't know much about your system, it's likely that it's a fairly minimal one (a "good" one takes time/money/effort, so you would tend to know more about it). Most boats come with a fairly minimal system. You also (apparently) have no direct way to measure what you have and what you are taking out (that would be a battery monitor such as the Victron BMV 700S or similar). You can still approximate it by figuring out the numbers, as Tom said.
First of all, what is your battery setup? How many batteries do you have, and how are they allocated? What are your options for isolating one to start your engine (i.e. it is sacred and untouched while you are at anchor)?
Next, going with the food analogy, you first need to figure out "what is in the pantry." That is, what size/capacity are your batteries? There are a certain number of amp hours (food) in every battery. One thing about batteries that is different than food, is that if you drain them completely you either kill them or drastically reduce their life span. So what you want to do is only ever drain them to 50% at most. So right away, if you have a 100 amp hour battery, and even if it is full (it probably will not be), you have only 50 amp hours or less. Just for example, a small refrigerator may draw 3 amps when it is running. It doesn't run 24/7, but say it has a 50% duty cycle. That is 36 amp hours in a 24 hour period.
So okay, say you have two moderately sized batteries and each has around 85 amp hours when full. Okay and say they are full when you arrive (outboard engine alternators don't always charge to full, but let's say you were plugged in and charged up that way before leaving). So now you have one 85 amp hour battery to work with, and you can discharge it 50%, which leaves you 43 amp hours to use. Right away you can see that the refrigerator is going to devastate your "pantry."
If you have incandescent light bulbs they may draw an amp or two each hour when on, so limit those (LED draw less). Everything you own that is electrical has a specific draw that you can figure out (look at the appliance; you may have to convert to amp hours from something else, but that is easily done online). The two big "eaters" are things that run for long periods of time (cabin lights, refrigerator, anchor light, computer), or things that just draw a huge amount (things that produce heat often draw a lot). On the other hand, for example, my horn draws 5 amps, but who is going to use that for more than five seconds?
Okay, now to simplify: Presuming you don't have anything "special" in the battery/charging department (i.e. you or a previous owner have not purposely gone out and put in a spiffy/expensive/modern system), then you probably have minimal amp hours in your "pantry," and you likely have light bulbs and other things that are "hogs." You also likely have no way of knowing what is IN the pantry to start with, or how much "food" is left as you go. So you are somewhat working blind (you can calculate usage, but you can never see exactly what you have used or what you have left).
In this situation, I would go completely "camping" or "sailboat in the old days" mode. Headlamps or battery powered lights for daily use (LED battery lantern on table). Very minimal charging of cell phones, etc. And probably a cooler with ice and not the refrigerator.
When I was on Powell last year, I had one 105 amp hour house battery (this is a bit larger than typical house battery), an isolated (different) start battery, and a battery monitor. I used mostly headlamps and battery powered lanterns, carried a cooler with ice, and really the only battery power I used while at anchor was for an LED anchor light, and to occasionally charge my laptop or my buddy's phone or tablet. Otherwise we just didn't use power. We didn't have that much to spare. I did have the battery monitor so was able to see how it was going. I was able to keep it above 60%, so that was good. But I don't think I could have run a refrigerator without either a huge battery bank, or a means to re-charge it while on the hook (solar panels, etc.). The engine does charge when underway, but at least mine doesn't have a very big alternator, and it is not very good at charging the "top" 15% of the battery (so it's very possible to stop for the day without the battery being up to 100%).
If you can get back to use with more details on your setup, we can be more specific.
On the other hand, my short simple answer (heh), if you don't want to go to a lot of trouble figuring things out right now, is to assume you have a minimal battery/charging setup, and go with headlamps, battery powered lanterns, and a cooler. Then you can still charge up electronics, and use the occasional cabin light, plus your anchor light, and you probably won't "kill" or adversely affect your battery (presuming you are not going to be at anchor longer than a couple of days).
