How long would a human have to spend in zero gravity to no longer be able to return to Earth?

Question

In the early days of space exploration, the crew of a prospecting mission gets stranded in the asteroid belt when their ship malfunctions. In addition, the group that sponsors the expedition has a tight budget, so the ship is not equipped with any centrifuge equipment. The ship has in-situ resource utilization capabilities, but by the time the crew fixes their drives/reactor, they have spent so long in zero-g that their bodies can no longer adjust to Earth's gravity.
How many years would the crew of this ship have to spend in space?

Answer 1

We don't know, and probably wont till someone dies after returning...

So there's two extremes to this, and I'm going to start by laying out the extremes:

Extreme 1 - Worst case:

• They're unfit / unhealthy to begin with.
• They're diet is missing vitamins and minerals.
• They do no exercise the entire time they're away.
• They're not under any medical observation the entire time they're away.
• There is no advanced health care available to them when they return to help them rehabilitate.
• They're not going to be rescued from their landing pod - they have to stand up on their own and resume life immediately.
• The "pass/fail test" starts from touch down. If they can't get out of the pod on their own, it's a failure.
• They have no support after returning, so if they can't hold down a job due to a disability they gain from their return, they starve and die, and thus fail.

Extreme 2 - Best case:

• They're the picture of health to begin with.
• They're eating very healthily.
• They're spending multiple hours per day on a treadmill or other equipment..
• There are doctors monitoring their vitals while away, or they're at least getting check-ups and observations.
• They're is epic health care waiting on Earth for them at no cost.
• There's a rescue team to help them from the moment they land.
• They can spend months or years in physical therapy and then declare their return successful.
• If they're unable to regain full mobility, they have a modern health system to help them with their new disability sufficiently that their return can be classed a success even if their confined to a wheelchair for life.

The setting of your question will vary between these two extremes, so will depend on the exact details of your plot. If worst case applies - it could be as little as 2 weeks - you loose (worst case) 20% of your muscle mass in the first 5 days in space. Extrapolating linearly that to 50% loss in strength over 12.5 days (worst cast) - which can be approximated as doubling your own weight. That link gave a best case of 11 days for 20%, which is still a 50% loss in strength extrapolation in 27.5 days.

If I'm landing a spacecraft carrying double my own bodyweight I'm unbuckling the seatbelt and immediately faceplanting on the ground. I may starve to death there.

At the other end of the spectrum, we don't actually know the upper bound. Valeri Polyakov spent 437 days under a scenario very close to the best case, immediately after landing, he was able to walk a few feet, he also seemed to be alright long term:

He also underwent two follow-up examinations six months after returning to Earth. When researchers compared the results of these medical exams, it was revealed that although there were no impairments of cognitive functions, Polyakov experienced a clear decline in mood as well as a feeling of increased workload during the first few weeks of spaceflight and return to Earth

... and 25 years later, apparently "Polyakov is currently the Deputy Director of the Ministry of Public Health in Moscow" - so there was no long lasting effects, so clearly the answer is more than 14 months.

Going off that Wikipedia article, your bones are decaying at a rate of about 1-1.5% every month in zero gravity. We also don't know if supercharging the fitness program in space slows that decay down - I suspect it will to some degree. Lets call it 0.5% per month if you're doing 5 hours in the gym each day. 0.5% loss per month done month-on-month is 5.8% per year, 45.2% after 10 years. 83.5% after 30 years. I suspect it'll level off before zero as your exercise regime will help a bit with bone metabolism, but your skeleton disappearing is a big issue.

In any case, returning with osteoporosis and multiple broken bones to a competent modern medical system that can repair the damage is still returning successfully. Living after a hip replacement and with a few bolts in your limbs is still living.

... but if I'd had to guess (assuming best case extreme), the answer is psychological, not physical.

A fit, healthy, 25 year old, stuck in space for 50 years in zero g, but with a decent diet, an uplink to a doctor, doing multiple hours of daily exercise, returning to Earth at 75 all frail with a rescue team, with modern medicine and a caring health system, would have a decent chance of living his golden years on Earth. He may be wheelchair bound, he may be dependant on a disability payment or old age pension, but that's still a successful return.

But I wouldn't want to return to that after 50 years in space - I'm mobile and can fly and can throw heavy objects clear across a room. The thought of leaving my happy floating realm and slowing rotting in an old folks home suffocating under my own weight just isn't appealing. I'd probably reject the offer of rescue unless there was immediate danger.