How many times or days does a green card holder have to visit the U.S. and how many does one have to stay in U.S. on each visit?
Short answer: There is a difference between “continuous residence,” which refers to a requirement for naturalization, and maintaining residence in the U.S. for green card purposes:
If you are interested in obtaining U.S. Citizenship as soon as possible and become a naturalized U.S. citizen, then as you receive conditional green card and enter country, you should aim to spend at least 180 days per year in the U.S.
If, on the other hand, you are interested in simply maintaining residency status and your green card, as long as you spend no more than 180 CONSECUTIVE days OUTSIDE of the U.S., you may fulfill this requirement. All decisions relating to maintenance of the green card should be discussed with an immigration attorney.