The rate of "normalization" would vary person to person. Hence why ISS personnel have mandatory exercise in space so they won't be essentially crippled when they return to earth. The exact statistics are unknown but it is reasonable to assume without countermeasures eventually moon gravity would feel similar to earth gravity eventually. IMHO
IMHO is related to no one being in space for an extremely extended period of time and very few people for any extended period. This leaves too small a pool of data to draw anything other than cursory data. The longest time by anyone was Valeri Polyakov at 14.5 months (almost 438 days). I am a retired Registered Nurse and do have extensive knowledge of Anatomy & Physiology. This bone loss begins within the first few days in space.
The most severe loss (of bone) occurs between the second and fifth
months in space, although the process continues throughout the entire
time spent in microgravity. Extended stays on Mir have resulted in
losses of bone mass of as much as 20% (indicating there is a range).
Astronauts regain most of their bone mass in the months following
their return from space,but not all...
Ref
Calf muscles biopsies before flight and after a six months mission on
the ISS show that even when crew members did aerobic exercise five
hours a week and resistance exercise three to six days per week,
muscle volume and peak power both still decrease significantly.
(Actually posts study on webpage as well.) In the study they measure muscle strength pre- and post flight in conjunction with biopsies of the soleus and gastrocnemius muscles to measure cell size and the structural changes of fast and slow muscle fibers. Chemical analysis measures muscle fiber structural changes involving myosin, (drives muscle contractions and cell divisions, enzymes, and substrates.) Electron microscopy "determines the relationship between thick and thin filament, the amount of myofilament loss, and changes in membrane-associated protein complexes found in skeletal muscle fibers and connective tissue that help the muscle resist stretch-induced damage."
MRI studies are done as well. These tests are run at multiple intervals both before and after space flight. It goes into further detail but shows significant variance in muscular atrophy, especially with a small sample set. A short synopsis reports
overall calf muscle volume significantly decreased by 13¬+2%, peak
power decreased significantly by 32%, force-velocity characteristics
were reduced significantly to -20 to -29% across the velocity
spectrum, and there was a 12-17% shift in myosin
(As the atrophy progresses, strength will most likely continuously decline to the point where moon gravity would "feel" like earth gravity. (This would be the reason for the IMHO part of my conjecture. Almost all the data collected, and all data in this study, is over a period of 6 months. There is no data for a long enough period of time nor is there any data with the relative gravity of the moon. This data is for micro-gravity and we can only theorize as to how the data would change with the low gravity of the moon).
It should also be noted these are all individuals not only with the exemplary physical conditioning required to be accepted into a space program, but that this also indicates there is most likely relatively minimal variance among them in their level of physical conditioning. This is a small data set of individuals that are all in good health, meeting a high standard for their physical fitness and with relatively low age variance and yet has a statistically significant variance in muscle breakdown over the same time period in the same environment. If in only 6 months peak power decreases by 32%, muscle volume decreases by approximately 13% and force velocity decreases by 20-29%, I don't find it unreasonable to predict the eventual loss of "superpowers".
