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.
