One benefit of being a nudist is that we can use 100% of our bodies to make vitamin D, the best way to get this vitamin. But how much sun do you need?
It's not a simple question, but there's a tool to make the job easier:
Norwegian Calculator for Vitamin D from Sun

Scientists at the Norwegian Institute for Air Research have devised a calculator at https://fastrt.nilu.no/VitD-ez_quartMEDandMED_v2.html
This calculator will estimate how many minutes of exposure you need for your skin to produce 25 mcg (the equivalent of 1,000 International Units) of vitamin D.

It is not written for US cities so you’ll need to visit www.realestate3d.com/gps/latlong.htm to find the latitude and longitude and enter the numbers manually.
If your latitude is 39 S, enter -39. If  your longitude is 76 W, enter -76.


Altitude - the easiest way may be to simply Google "altitude of [your town]". Remember to convert it to kilometers. One kilometer is about 3300 feet. 

It asks skin type.
Here's a little more detail if you are not certain: https://beautisecrets.com/skin-tone-chart

Surface type is whatever you are reclining on. In my yard, the choices are the concrete patio or the grassy lawn, but adjust for your setting.

You’ll also need to enter the time of day you are going out in the sun, expressed as UTC (Greenwich Mean Time).

You can convert your time into Greenwich Mean Time by going to www.timeanddate.com/worldclock/converter.html.
The calculator uses a 24 hour clock, so hours from 1 PM to midnight are expressed as 13 to 24.

The calculator also wants to know the thickness of the ozone layer. I suggest just setting this one to medium.

Be sure  to click the radio button next to the entries. They are often not automatically selected when you fill in the values.

Keep in mind that the exposure times given are considered enough to maintain healthy vitamin D status.
If you are starting out with a vitamin D deficiency, you might need more.

This is not about tanning and you won't need to burn your skin, not ever!
It's simply about health.
You'll probably notice that late in the year, even if you can stand the cold, the sun is simply too low in the sky to make vitamin D,even if you are fair skinned and you live in Florida. The goal is to make enough vitamin D to last you through the "Dark Months". Not much time is left!
Get those clothes off and make some D!!!

Ramblinman:
We have a gut calculator, too, I think. We, untrusting and theorizing, like to rely on numbers, but our body pretty much tells us when we overdo something or when we need something.
We can pull out our slide rules, calculators, charts, maps, and formulas, but we were issued appetites, tastes, and cravngs to help us handle the weather and the terrain that we encounter.
I treat formulas as a report card, not as a recipe. Perhaps by then it's too late, but I like to trust the body that we were given to know how we're doing.
Relying on some formula which may not work for you can be hazardous.

VeryGary,

Sunburn is a quick reality check that you have exceeded what your body can handle for the day, but you don't want to get close to that point.
This chart is designed to help you come into shade before skin redness is obvious. You can't intuit your way through this.

Inadequate Vitamin D is also a health risk: In using this calculator, I saw that even in the Florida Keys, you cannot make Vitamin D in the dead of winter, so don't try for that. Find out when the UV B supply shuts off in your area and make sure that you have enough D stored in your body. A test in your doctor's office is the only way to know for sure, not some general sense of well-being. People have been shocked to discover low D levels. People who are extremely fair may have such delicate skin that only diet can safely deliver the D they need. For most of us, diet is a poor substitute for D derived from sunlight. The chart of skin types that I provided is a good adjunct to the chart on sunlight.

However, sunlight helps our serotonin/melatonin balance, and thus balances our mood. Again, you shouldn't take this as permission to blow past your body's limits for UV exposure. People with seasonal affective disorder need to see a doctor. A friend of mine moved to a sunnier climate because of it, but doctors can guide you through at-home treatment so you don't have to move. If you have the garden variety of wintertime sleepiness, just get out of the house a little more during daylight hours and you'll probably "wake up".

Nothing is inherently dangerous in these charts. They don't replace common sense, but I must disagree with the notion that your feelings are the only guide to sun exposure.

While most sunscreens come with hazards because they use chemicals that penetrate the skin and can cause the very cancer you are trying to prevent, sun blocks are "mechanical" photo-blockers and will sit on top of the skin rather than absorbing. In primitive camping, mud is the time-honored sunblock. City folks will shudder at the thought of using it, but it is better than a sunburn! After you've had enough sun, shade is another ancient remedy.