I realize the answer to this question varies with the time of day (most recommendations are to avoid exposure between 10am and 2pm), latitude, altitude, medication and other factors, and goes as far as claiming that "there's no such thing as a healthy tan".
But let's assume a white person in their 30s without an existing tan, who doesn't take any medication, living at around 35 degrees north latitude (California) at around sea level. For how long can they safely expose one unclothed body area to the sun, without sunscreen, outside peak hours, during the summer without clouds?
By "safely" I mean without non-obvious detrimental changes lasting longer than a week:
- sunburn is obvious and not safe
- serotonin increases are non-obvious, but not detrimental
- DNA damage that does not materially affect the individual but might be passed on to offspring is outside the scope (assume the individual has already reproduced, and/or no longer plans to reproduce)