No, the application isn't limited to a number of free days. It can be used forever, but after the first year, you will need to purchase it for another year for 0.99 cents. If you have trouble with the application, go to Settings > Applications manager > All > WhatsApp. Tap on it and select Clear data and Clear cache then reboot the phone and it should start again. Additionally, perform a fresh install using the Store application in your device.