I would recommend charging it for 30 mins to see if it turns on however it should be able to turn on after only 5-10 mins and should've already had enough power to turn on upon opening the box. If its not turning on try holding the power button (the long one on the right side) for 10 seconds to see if it shows the low power screen (a power bolt and the time in red). If it isn't showing anything even after being charged for 30 mins then I would recommend contacting Apple Support (https://getsupport.apple.com) and asking to have it checked or replaced.
For more help visit this support page: (If your Apple Watch won't charge or it won't turn on - Apple Support)
Feel free to reply to this message if you run into any trouble.
I hope this helps
-Person