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 inTax implications: When you sell the companystock, you work forhave to pay taxes on the profit (including that 15% discount). The tax rate you pay is based on how long you wait to sell it. If the company goes southyou wait a certain amount of time (usually 2 years, then the stock pricebut it will obviously dropdepend on your specific tax codes) before you sell, but you'll alsoyou could be at risksubject to be laid offlower tax rates on that profit. As such you're exposed muchSee here for a more risk than investing in other companiesdetailed description. This is a good argument to sell the stock and takemight only apply if you're in the 15% profitUS.
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 onI am generally wary of being significantly invested in the profit. The tax rate you pay is based on how longcompany you wait to sell itwork for. If you wait a certain amount of time (usually a yearthe company goes south, but itthen the stock price will depend on tax codes) before you sellobviously drop, you couldbut you'll also be subjectat risk to lower tax rates on that profitbe laid off. See here for aAs such you're exposed much more detailed descriptionrisk than investing in other companies. This is a good argument to sell the stock and take the 15% profit.*
* - I realize your question wasn't really about whether to sell the stock, but more for when, but I felt this was relevant nonetheless.