To make a bow for a cake do I have add something to the fondant or can I can I just use regular fondant and leave to to dry for a week.

you can use regular fondant, but it may take more than a week to dry depending on your humidity level.

If using fondant i suggest rolling it out as thin as you can though so it can dry quicker.

You need to add something to help it hold and to help it dry crispy like some tylose, candy clay or some gum paste to it.

I add cornstarch to help it hold it's shape and it helps it to dry out and it's cheap and effective, which is pretty efficient I think.

http://www.cakejournal.com/archives/how-to-make-a-gum-paste-bow/