I would suggest starting as soon as possible because the wait could be longer than the length of your H1B. Of course first you want to make sure that your employer is willing to file for a green card on your behalf. But as soon as you know that they are willing, you should begin the process. In other words, there is no real reason to wait other than not wanting to annoy your employer.
When you do feel like you are ready to start, I would suggest that you speak with an immigration attorney. The green card process can be tricky and an experienced immigration attorney will be able to guide you through it.
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