I would like to apply for green card for my mother and father. am a US citizen, they will be coming to US in next month on tourist visa. How long will I have to wait (day/months) after they land in US, before I can file for their green card.? Also, if law changes for so called "chain migration", can that law be retroactive for applications in the que? Any comments on that would be appreciated.
Your parents must not have a preconceived intention to file for a green card. I have covered this issue in a bunch of our frequently asked questions. Please take a look. In the USA, ex post facto laws are considered to be unconstitutional.