What are the minimum years required for the preparation of IAS while working?

It depends on your ability. You should be able to manage your work burden and you should also prepare for UPSC. Normally, it takes minimum of 1 year for preparation of UPSC. But once you refer to the whole syllabus of UPSC, you may decide for yourself.

Totally depends on a person. One can track it in one year or 6 months, but because of some reasons, so many didn't so just start working hard and be confident. Best of luck for your future.