urgent help need about parents

visit visa.

Hi, There,
I have H1B visa now and I want to invite my parents come to visit me. I’m about writing the invitation letter and there are several questions that I don’t quite clear.
1. About the invitation letter, is there anything I need to be aware of or I just go ahead and say I would like them to come and visit me?
2. My parents’ passports are about to be renewed (7 months left), will this affect them to get visa?
3. Is the immigration officer gonna ask the how long do they want to stay in US? If so, should they say 6 months or less?
Any information will be highly appreciated. I really need help.
Thanks