How much time in zero gravity is needed to cause the negative side effects of zero gravity like bone loss

Do the zero gravity flights cause bone loss to the pilot who flies these zero gravity flights every day a few times a day

Russian Cosmonaut, Valeri Polyakov, launched 8 January 1994 (Soyuz TM-18), stayed at Mir for 437.7 days. Upon return, he was given a physical and found to have lost some muscle mass and to have water retention. Astoronauts in long space missions now use stationary bikes and sometimes strap in treadmills to work on maintaining their body mass.

Short zero gravity flights are not long enough to have bone mass effects, but they due cause a temporary enlargement of the heart - a side effect that corrects itself after about two hours.

i think its a year and a half approximately.