Yes, it is particular to the contract and state laws. Most decent contracts state the employee will "dedicate appropriate time and effort to completing assigned

Yes, it is particular to the contract and state laws. Most decent contracts state the employee will "dedicate appropriate time and effort to completing assigned tasks". Of course this is vague, but putting in 10 hours a week is a sure way to get fired. If it turns out you were spending 30 hours a week on your startup at the same time (and depending on other company resources used, etc.), your startup work may not be owned by you. Most likely, you'll just be fired for non-performance but depending on the circumstances, your employer could sue for ownership. This would especially be the case if you went from Microsoft to Google or something similar.