Having a prototype is essential if you want investors to take you seriously. So good job on getting that.
It would be very good if you can also get a designer to also make 1 or 2 sleek representations of your vision for what some key screens of the app could eventually look like. These screens are just to have in a deck, and maybe you could even add some superficial functionality by using something like Marvel (https://marvelapp.com/). Feel free to add buttons, options, etc. to the screens which you haven't implemented yet in your MVP. It's just to show your vision for the future.
Then also have at least some data or insights to present from your preliminary user testing. Even info from just 5 user tests is better than nothing, but the more you have the better. You probably won't need anything more than 50 though (depending on what your app does), so you don't go crazy on that before talking to investors.
Start by 'pitching' your app to just friends first, then to strangers (maybe your neighbor, whatever), then to investors you're closest to (maybe you're connected to some on linkedin?), then to investors you don't know, etc. At each of these stages you'll be getting more and more confident about your product, you'll be working out bugs, and you'll be perfecting your pitch to get the most interest. By doing it in steps you're not putting all your eggs in one basket, and you'll instead be increasing your chance of getting investment as your approach bigger and bigger investors.
If you want to run your current prototype by me for suggestions, inquiries, etc. send me a message and we can set up a call,
all the best,
I believe the best time to approach investors is when you have working a model; as far as closing a deal.
However, it's good to run your ideas by investors and let them ask you questions that you're not ready for. That way you can continue refining your idea and your approach.
Does your idea have traction? Are there users or customers? Are people getting excited about what you're doing?
All things to factor in when determining the best time. At the end of the day, when in doubt pitch now. What's the worst that could happen? I'd recommend before you start pitching investors to start pitching entrepreneurs who are strangers and see what you get.
Hope this helps.