Unlike public companies, updates are not required by law, but you should plan to let your investors know about your growth, sales, new partnerships, hires, and the other things going on at the company at least quarterly.
Not all of these updates need to be long or serious – even just posting a few photos or paragraphs detailing the new cool thing your company is doing will show investors that you're using their money to help move your company along. The better they feel, the more likely they are to help when they can.