Well, if you want to be detailed, you'll want to find out your skin type, latitude, and time of year. I've seen charts and based on them and from them it's possible to figure out how long to stay outside with out burning. Basically stay out in the sun long enough so that you do not burn your skin. Avoid wearing sun screen during that time. Once you reach the point where a sun burn will develop slather on sunscreen or head indoors. With that, you'll slowly build up a tan.