Yes, you can. There's even a noun for this: 

les quinze jours -- a fortnight.

Here's a related oddity worth knowing:

Say it's Monday and you want to ask a friend if she has time on Friday the following week. In French, you would say: 

Tu es libre vendredi en huit? -- Are you free Friday next week?

This just goes to show that counting days isn't mathematically correct in these circumstances.