How long should you keep your idea a secret?

I am a graduating senior in college studying computer science and machine learning. For the past year, I have been working with two other people on what we think is a pretty fantastic idea, offering a service to people that they didn't realize they needed.

We are almost to the point of having a working prototype (web/software).

Up until now, we have been very secretive about our idea. We've given some people a vague idea of what we're doing, but we're afraid of being too vocal about it because we don't want someone else stealing our idea.

At the same time, however, I feel like we should be getting the word out for marketing purposes and user-research to see if people will actually use the service. Of the few people with whom we've talked, everyone has been very enthusiastic and expressed a lot of interest, saying things like "I would have never thought of that, but that would be awesome to have".

In addition, I'm not sure when it is appropriate to start approaching investors (which I know nothing about). I think this is along the lines of the first half of the question, so I'll leave it here.

Thank you in advance for any guidance you can offer!

EDIT: Regarding the idea, it falls in the education/academics sector, and our anticipated revenue stream would come from colleges and universities.

If you haven't talked to people about it, how do you know it will solve their problem?

Regarding someone "Stealing your idea", there's a couple of things you should consider:

1. If it's that easy to steal, it's going to get stolen the second you launch
2. Most people are more interested in their ideas than yours
3. If you thought of it, someone else has almost certainly thought of it too
4. Even if someone steals the idea, they can't steal your vision and ability to execute.

Remember, Google was not the first search engine, the iPhone wasn't the first cell or smart phone.

Talk to as many people as are willing to listen. Listen to their feedback. Write it down. You will gain invaluable insight and advice.

If you are in fact close to having a working prototype I would encourage you to focus on finishing it before you spend too much time talking about it. Once you have something that works, you are at a major advantage compared to someone else who might take your idea and build their own solution.

To understand why, you need to understand time to market.

In commerce, time to market (TTM) is the length of time it takes from
a product being conceived until its being available for sale. TTM is
important in industries where products are outmoded quickly. A common
assumption is that TTM matters most for first-of-a-kind products, but
actually the leader often has the luxury of time, while the clock is
clearly running for the followers.

So lets assume some big company with lots of cash and expertise (like Google or Microsoft) learns of your idea and loves it. In fact they love it so much that they want a product that does that. They will sit down and ask, what would it take us to make that product? Now they have no shortage of talent or money, but they don't have any more time than anyone else. Lets say they decide it will take them 6 months to build and launch the product. That is a 6 month head start you have on them (assuming your product is done), and a lot could happen in 6 months. They will also be thinking what happens if these guys sell out to IBM while we are building our product and IBM puts big marketing bucks behind it?

That is why a big company like Google or Microsoft would much rather just buy your product from you if they want it rather than take the risk of building it in-house. Generally building the product from scratch is the least desirable option. That is why you see companies like that buying other companies all the time.

Now if you talk about your idea before it is ready, and someone else can build it before you can finish it, then you have lost the advantage of being first to market.

Assuming you have no patents or other ways to keep your work proprietary, right now secrecy is your friend.