I am working in USA under H1B VISA. Now I am planning to apply Green Card through my Employer.
But my question is is there any minimum and maximum time that I have to spend on H1B in order to apply Green Card in USA? If yes what are those time limits?
There's no such minimum. Your employer can apply for an immigrant worker even if that worker hasn't spent a day in the US.
There's no maximum either, but there's a maximum limit on your stay in the US on H1b status. If you reach that limit without applying for permanent residency - you'll have to leave the US for at least a year. If you're in the process already - your H1b can be extended, despite that limit, until your AOS process is complete. There are certain conditions under which such an extension can be granted, and one of them is how long you've been in the employment-based green card process already. It is best to start your green card process at least a year before the 6-year H1b limitation expires.
There are many benefits to applying for your permanent residence (green card) as soon as your H1B is approved. Most notably, you can reuse your labor certification which should reduce the amount of work that your lawyer has to do (and maybe, just maybe they will lower your fees). There's really no disadvantage (except financial) to getting this done as quickly as possible.
