I would tread lightly here for the simply fact that you certainly would not want to be placed in a position where you are forced out of your current position prematurely. As the previous posters have correctly stated if you are an at will employee, your employment can be terminated if your employer believes that your outside venture is interfering with your current employment.
However, if you have an employment contract or have what is considered a quasi contract that could be construed as implicitly outlining the conditions under which you can be terminated it is important to consider if you have run afoul of any of these “for cause” reasons for termination.
Ultimately this becomes an issue of personal risk tolerance and how wedded you are to remaining with your employer. If you need the money from your current job I would advise being transparent about your intentions in that the startup is a long-term plan for you and that eventually you will be devoted all of your time to it. Try to be accommodating as possible and remind them that while you are at work you are completely focused on the job at hand. Further, it may be beneficial to you to also remind them that when the time comes you will provide adequate notice and assist in smoothing the transition. If you experience push back try not to burn bridges and make sure all of your bases are covered.
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