Short Answer: Weeks to months, depending on complexity
This is hard question to answer because of all the variables, but I'll do my best to give you an idea based on my experience.
The first app that I built, which was pretty simple in the beginning, took about a weekend and a couple days.
This app consisted of:
Another app that I just finished took about 6 weeks, part time. A more complex app.
All of my Shopify App dev is solo, so you can use that as a guage. Want it faster, hire more devs or cut down on the complexity.
Design is relatively easy with Shopify since they give you a styleguide called Polaris. If you create a complicated UI, it will add complexity and development time to your app.
The submission process is a bit of a wildcard. This last app it took two rounds of edits and less than a week. Follow the directions and you can sort of control this time.