The fair answer is "never." If we were to use a "common sense about how the world works" approach to decide what claims don't even merit an experiment, scientists would probably never have discovered quantum entanglement (for example). Still, I think there are times when laws of science have been so well established, have been verified in such a myriad of ways, that anyone who says they've discovered an exception probably deserves nothing more than a dismissive laugh. (This probably explains why the United States Patent Office will no longer even read submissions for designs for perpetual motion machines.)
All of which brings us to a 65 year old spiritualist named "Naveena" who claims she is living on nothing but light for nourishment.
She's not the first to make such a claim. There's an Indian yogi named Prahlad Jani who claims he hasn't eaten or drunk anything in seventy years. He allows himself to "dip in water" and sometimes "gargle," but he doesn't swallow. Ever. Another, who calls herself "Jasmuheen" but whose legal name is Ellen Greve, is an Australian who wrote a book called Pranic Nourishment: Living on Light. Jasmuheen participated in a test of her claims, which had to be discontinued after 48 hours because she was on the verge of going into shock from dehydration. About four of her followers who died, she denies all responsibility, and in fact said about one, "(Lani) Morris was not coming from a place of integrity and did not have the right motivation."
So now Naveena is going to try the same thing. Maybe her "motivation and integrity" are better.
"As early as 1910 Richard Steiner, scientist and philosopher, proclaimed that 'Matter is Condensed Light.' Breatharians, Sun-gazers and Yogis have claimed the capacity to 'Live On Light' for centuries but without hard evidence their claims have been dismissed and ridiculed," Naveena said in a press release last week.
Yes, well, Steiner may have said that, but basically, he's wrong. We now have this thing that explains the properties of matter and energy, and how they interact. It's called "physics." Recommended that you take a course or two in it.
Of course, she admits that things might not work out for her.
"Since death is what normally occurs when a person does not eat food, she warns people not to try this," her press release stated. "It might be untrue or it might be that there are certain criteria needed in terms of physical, mental and spiritual readiness."
And that's just it, isn't it? If you can fall back on blaming the practitioner for the failure -- as Jasmuheen did with the death of Lani Morris -- you don't have to admit that the whole claim is a sham. As far as Prahlad Jani, I'd bet that he's simply lying. There are organisms that can survive on light -- they're called plants. Animals like us, on the other hand, don't get by so well unless we're allowed to nosh every so often. So unless Jani et al. can show me that their cells contain functional chloroplasts, I'm not buying it. (Here's the Skeptic's Dictionary's take on Jani, which outright calls him a fraud.)
Still, I guess Naveena has the right idea. At least she's willing to participate in a test. She's got a YouTube channel, from which she is broadcasting updates. The problem is, given the extraordinary claim we've got here, just periodic film clips documenting her progress aren't going to be enough to convince me, and (I suspect) most people; if what Naveena is doing is real, I'd want some round-the-clock footage, that she had no way of controlling, that would assure me that she wasn't periodically sneaking out for a cheeseburger.
So that's today's dip in the deep end of the pool (just immerse your toes! No ingesting the water!). My guess is that one of two things will happen, in Naveena's case; (1) she'll discontinue her experiment, voluntarily, at the advice of her doctor; or (2) she'll be exposed as a hoaxer. Either way, the "breatharians" and their "living on prana and light" will remain a fringe claim, meriting nothing but a shrug and the comment, "sorry, the world doesn't work that way."