I've personally water fasted (nothing but water, salt, and potassium - zero calories) for 12 days and it was only beneficial, not detrimental to my health. It also wasn't all that difficult since my body was used to shorter fasts already. I've known people who have gone significantly longer and 30 days isn't extremely uncommon. Like you mentioned, a lot depends on how much fat a person has stored up, but even someone already at their ideal BMI could fast for 30 days without any lasting negative consequences if done correctly.
I've personally water fasted (nothing but water, salt, and potassium - zero calories) for 12 days and it was only beneficial, not detrimental to my health. It also wasn't all that difficult since my body was used to shorter fasts already. I've known people who have gone significantly longer and 30 days isn't extremely uncommon. Like you mentioned, a lot depends on how much fat a person has stored up, but even someone already at their ideal BMI could fast for 30 days without any lasting negative consequences if done correctly.