This is an excellent (if unanswerable) question.
There is insufficient evidence to provide an answer which meets SE standards. But you asked for an educated guess. I’m educated. Here’s my guess:
First, the question requires some qualifiers to reduce it to a bite-size chunk.
You need to specify if you are including re-adaptation to Earth gravity. Some health effects of low gravity (muscle atrophy, decreased bone density) could be considered adaptive in that they are only maladaptive on return to higher gravities. Maybe a bunny-hopping Lunar Citizen doesn't plan to return to Earth and is not concerned about muscle atrophy. And if the subject must re-adapt to Earth gravity, what do you consider a reasonable period for re-adaptation? If it takes 10 years to regain bone mass (during which the subject is at risk of fractures), is this “reasonable”?
Does your definition of “healthy” include treatable health effects of low-g? For instance, calcium loss from bones will increase the risk of urolithiasis (kidney stones). The risk can be reduced by diet. Both medical and surgical treatment is available. Ask someone with renal colic if they feel “reasonably healthy”.
You ask about long-term health. How long? In the long term, we all die. Do you mean a small decrease in life expectancy? Say, maximum 5 years of life lost? (By comparison, smoking shortens life expectancy by 10 years). Let’s say the heart muscle atrophy produced in low gravity results in a 1:10 chance of early cardiac death and a 2 year reduction in life expectancy. Do you consider this reasonable?
An apparently healthy person with a compromised immune system https://www.ucsf.edu/news/2021/06/420756/space-travel-weakens-our-immune-systems-now-scientists-may-know-why and living with higher mutation rates in microorganisms https://pubmed.ncbi.nlm.nih.gov/11027966 may appear “reasonably healthy” but have increased risk of infectious disease.
One of the worrisome effects of microgravity is Spaceflight-Associated Neuro-Ocular Syndrome (SANS) https://en.wikipedia.org/wiki/Visual_impairment_due_to_intracranial_pressure#Non-invasive_ICP_measurement . This condition, found in 15 long duration astronauts, shares features with Idiopathic Intracranial Hypertension. The cause of these conditions is unknown, but anything amiss inside the brain is worrisome for astronauts (pre-retirement, anyway).
So, here’s my educated guess:
For no Earth return, allowing for an acceptable 5 year loss in life expectancy and allowing loss of cognitive functioning after age 65, I’d guess 0.5 +/- 0.4g.