The longest time anyone has spent in space is just over fourteen months. So far, astronauts have recovered surprisingly quickly, even after the longest duration spaceflights. Within a few weeks they are almost back to normal health. There are some longer term effects, on load bearing bones, which may take a couple of years to clear up, and very rarely, permanent effects on the eyes.
But what about longer periods in space than that? Their condition continually deteriorates for as long as they are in space and so far nobody has spent as long as two years in space; the record is fourteen months. So the question is still wide open for longer duration flights.
First let's look at what we know already.