I drive about 5 - 7 miles a day on average - thus, it’s concievable that a full charge once a month is sufficient for me
No, its not conceivable that a full charge once a month will be sufficient for anyone with one of these cars.
If you drive 7 miles a day that will likely be 15 or even 20 "miles" lost off the cars range meter every day, not counting any battery drain while parked. Lets examine exactly how often you will have to re charge (it absolutely, positively 100000% is not "once a month".
I have posted this more than a few times, but this is, in my opinion, the single biggest mistake prospective tesla owners make (" oh, I dont have home charging, but I only drive X miles a day, and the battery is Y big, so I will be able to charge only once every 2-3 weeks and have plenty left over").
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A US model 3 (which used to be called Model 3 SR) has a EPA range of 267 miles when brand new. "EPA" range is important because you wont ever achieve that, but thats what we have to start with for looking at how much you might "actually" need to charge.
Even if you get a LFP model 3 and its recommended to charge to 100% (at least once a week per teslas information), Charging to 100% at a supercharger is going to take you quite a bit of time because of the charging curve at the top. It slows down as you get near full, when supercharging (but not when home charging because ALL home charging is "slow charging").
Anyway, Even if its a LFP model 3 and you start with EPA range 267, you are not going to run it down to 0, no one does that in an EV. You are at most going to run it down to, on average, around 20%. 20% of 267 is 53.4 (lets call it 54), so what you will actually be calculating as usage is 100% to 20%, or 267 - 54 = 213.
Ok, now we know that you have 213 miles of range you will actually have to use on a regular basis (to start). Your 7 mile daily commute, being so low range, will likely use 15 miles of range from your car, maybe even more in the winter for starting up, normalizing cabin temperature, etc. Lets use a fairly conservative number of your 7 mile commute using 15 miles of range (which will be mostly accurate, except it will likely be more in the winter).
You will be losing 15 miles of range from your battery every day, and you have 213 miles of range to use (see above). Thats 14 days (not a month). This does not count any usage you might actually have sitting in the car waiting for someone, etc. Nor does it count SENTRY mode, which, people who park outside and dont have home charging almost always want to use.
Sentry mode uses between 1 and 2 "miles" an hour. Lets use 1 mile per hour as a conservative estimate, and you parking outside at your home for 9 hours a day, and sentry mode of 1 hour used "elsewhere" during the day for a total sentry mode usage of 10 hours a day, and 10 miles of range used.
Add that 10 miles of range sentry mode uses, to the 15 miles of range your car will use on those short trips, and you have 25 miles of "range" used a day for your 7 mile commute, with sentry mode being used. 213 miles to use, 25 miles a day used, for an actual "I need to go fill up" time of about 8 days.
TL ; DR -- If you use the car like almost everyone else who does not have home charging and turn on sentry mode, you will need to "fill your car up to 100% full" every EIGHT days (and be at a supercharger over an hour to do that), no where near "once a month".
If you DONT use sentry mode, EVER and you use the car, you will fill up every 14 days (still no where NEAR once a month". There is no usage of the car you will do that will have you fill up once a month. If thats your expectation, dont buy the car. You should not buy this car unless you are OK with the fact that you WILL be spending up to an hour at a supercharger (or even longer if its L2 charging) every 8-14 days.
Back to the question you specifically asked. If you DONT use sentry mode, you will lose 1-3 miles of range a day if you dont drive the car and DONT use sentry mode. If you DO use sentry mode, you will lose approximately 1 mile an hour of range for every hour you use sentry mode, so in a 24 hour period of being parked (not touching the car at all) you will lose around 15-20 miles of range, when using sentry mode.
So, if you use sentry mode and dont drive the car for 7 days, your car will have lost somewhere between 140 miles range, and 200 ish miles of range (depending on if you have cabin overheat protection on, as well). This doesnt count you actually driving the car. The reason I keep mentioning sentry mode, is people without a garage are usually the ones who want to use sentry mode.