You can use it for 90 days, but after you need to buy a license. Since the evaluation is enterprise version you can't activate with a home/pro license, but need a full reinstall from appropriate media. I'd recommend buying a valid license straight, will save you a lot of hassle later on with the reinstall.
When you install windows on your build you will be able to use your PC I think for 30 days. But you will be restricted from doing certain things such as changing the background. I believe that once the 30 days is up you are forced to enter a key.
You can use it for 90 days, but after you need to buy a license. Since the evaluation is enterprise version you can't activate with a home/pro license, but need a full reinstall from appropriate media. I'd recommend buying a valid license straight, will save you a lot of hassle later on with the reinstall.