how early can you apply for stem opt extension
You want to apply for your H1-B as soon as possible! The point of the OPT + STEM extension is to give you time to apply for your H1-B visa. While you are employed under OPT, your employer can petition for your H1-B work visa. If you don't get it on the first chance (the H1-B visa is based on a lottery system), then you can continue working under OPT + STEM and apply again the next year. If you run out of OPT + STEM and don't have your H1-B by then, you can no longer continue working and must leave the country (which is a situation I'm assuming you don't want to be in).