Due to research done on mice on the ISS, we have some idea:
"0.33g mitigates muscle atrophy while 0.67g preserves muscle function and myofiber type composition in mice during spaceflight"
https://pmc.ncbi.nlm.nih.gov/articles/PMC12985678/
Obviously, we know that the gravity of Earth is sufficient.
But the results make probable that two thirds of the gravity of Earth might be enough, while the gravity of Mars may create some problems and the gravity of the Moon is very likely to be insufficient, so the time spent on the Moon must be limited, though not so much as on the ISS.
I agree with the previous poster that any spaceship designed for carrying humans to Mars or even farther must be designed to spin and anyone who accepts to go on something else is stupid.
Making a spinning spaceship may be cheap if dual bodies or one body and a counterweight are used. It is likely that the safest solution would be to have 2 identical spacecraft, which could also be used independently but which could be coupled with cables to spin around the common center of mass at a distance big enough to create enough gravity at a low rotation speed.
The problem is not the price but the fact that nobody has tested how difficult is to control such a configuration (avoiding oscillations and instabilities) and how difficult is to solve problems like docking in a manner that does not waste energy (i.e. without changing the rotation speed of the more massive spinning spacecraft, which can be done by having 1 or more docking ports on the rotation axis, like on the hub of a wheel; in the case when the rotating spacecraft would be made with 2 bodies or a body and a counterweight that would be linked with cables, one could have the equivalent of an elevator for transporting crew and equipment from the docking port to the main body or bodies).
But someone must build and test such a spacecraft, otherwise we will never learn how to do it right and which are the real problems that are hard to predict in a simulation.