When you're bringing in enough projects to pay their salaries. If you're planning on footing the bill yourself, then make sure you have enough cash flow to pay for this. If you're planning on working with them directly, make sure you're capable of managing the workloads.
We have a very similar problem. There are 2 developers in our company and we looked for ways to give work to other developers whereby the cost and time of explaining how to implement the code didn't exceed the time to do it ourselves. We experimented with ODesk contractors, which we found to be the best of the contracting web sites. Apart from a few peripheral tasks, outsourcing didn't work particularly well. We have very high coding standards and contractors generally don't. The exception is when you pay full Western rates for high quality contractors. But if you're a student, you're unlikely to be able to stomach these rate.
My suggestion that a commercial level is to work out exactly how much profit you make from each of the projects you have ongoing, then just dump the least profitable to make room for more. By brutal about your own efficiency first, before looking to expand (quality before quantity if you like).
First, pick a project and go with it. Develop it yourself unless you have a good friend who wants to help, a co-founder of sorts.
Keep the project simple. Don't develop too much. Just make it so you can launch your product with just enough features to make it useful. After that, you can focus on gaining customers and feedback from them.
From there you can begin developing more features and consider hiring other programmers. This is of course assuming you can afford to pay them.
I think there are two camps to consider here:
1) Do you want to build the ideas yourself? This has benefits like learning, decreased cost, but downised such as length of time. A lot of small businesses start out this way (This is my preferred method)
2) The 4-hour workweek method - just outsource everything up front and then sell the result. This has benefits of faster time to market and less personal involvement (you are freed up to focus on marketing, customers, etc) and the downside of higher expenses.
Pick a preferred development method based on how you feel and your goals, then go with it. I will also mention that for number 2, you'll probably want to invest a few thousand dollars for a product. For a good example of how this might work in practice, check out this interview:
http://mixergy.com/free-apps-interview/ (be cautioned that this is not a typical result and you are just as likely to see nothing come back from your investment)