An instant 15% profit sounds good to me, so you can't go wrong selling as soon as you are able. Here are a couple other considerations:
I am generally wary of being significantly invested in the company you work for. If the company goes south, then the stock price will obviously drop, but you'll also be at risk to be laid off. As such you're exposed much more risk than investing in other companies. This is a good argument to sell the stock and take the 15% profit.
Since you work for the company, you may be privy to a bit more information about how the company is run and how likely it is to grow. As such, if you feel like the company is headed in the right direction, you may want to hold on the the stock for a while.
Tax implications: When you sell the stock, you have to pay taxes on the profit. The tax rate you pay is based on how long you wait to sell it. If you wait a certain amount of time (usually a year, but it will depend on tax codes) before you sell, you could be subject to lower tax rates on that profit. See here for a more detailed description.