As soon as you receive your green card or the approval of your adjustment of status (I-485), you are technically free to change job, work for whoever you want, or not work at all. There is no established rules that prohibit you from "quitting" your green card sponsoring job. However, from an immigration prospective, it is not advisable to do so immediately after your green card approval. Given the fact that your employment based green card is supported by a permanent job offer, meaning you as the sponsored employee were intended to work on a more permanent basis for the sponsored job. By changing jobs or quitting immediately or a few months after you receive your employment green card, you may have a risk of a misrepresentation charge during your naturalization application, should you decide to become a U.S. citizen in the future. As a rule of thumb, you might want to wait 6 to 12 months before leaving your job as both you and your employer (during the employer sponsor green card application) were saying it is a permanent job offer under the terms of I-140.