When is the right time to start marketing?

Day 1 of when you decide to build something. Remember marketing is more than just Promotion. There are 4 Ps in marketing, Promotion is just one. The other Ps are Product, Price and Placement (distribution). 'Marketing' is very much about the product you're building. Speaking to potential users to understand their pain points is part of marketing, putting up a landing page for validation, is part of marketing. My guess is your question is more about 'When is the right time to start promotion?' rather than marketing though.

If you are alone, I would say after building your app. You won't be able to couple the development of your product and be efficient at marketing your product (except you got some insane brain power). If you got someone in your team dedicated to marketing, it's better to do it during the building process of the product. During the development, a lot of iterations will be taken and you want the marketing to still be relevant. That's where you also got to be able to share your ongoing product to get feedback as well. I don't think it's ever a good idea to market before building. There might be some context where it's relevant but I haven't seen this except in a business incubator environment.

Marketing is not an option. If you don’t market your business, you don’t have a business. It’s that simple. If you don’t market your business, you will be left to the mercy of chance, and if you think that chance is on your side, then you are going to be disappointed more often than not.

Ideally you want to start as early as possible, so before building and try and slowly ramp up to the launch of your first MVP.