How often and for how long can a US immigrant leave the US for abroad,and will it affect his file for citizen?

Answer 1

Here is what you need to know about keeping your permanent resident status; be careful, DHS is far stricter now about residents who stay away for too long.

http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=fe17e6b0eb13d010VgnVCM10000048f3d6a1RCRD&vgnextchannel=4f719c7755cb9010VgnVCM10000045f3d6a1RCRD

Also, here are the requirements to file for citizenship, including information about significant visits outside the US:
http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=12e596981298d010VgnVCM10000048f3d6a1RCRD&vgnextchannel=96719c7755cb9010VgnVCM10000045f3d6a1RCRD

Answer 2

A US immigrant must only stay outside the US for a short period of time in order to qualify as a US citizen because staying outside the US for long period will mean that an applicant is not really an immigrant.

Answer 3

If you're going to be gone for awhile, you can file for a re-entry visa.

Above all, you need to make sure you continue to file U.S. income taxes or else they'll assume you've abandoned any intent to stay in the U.S.