Always.
Faith is what prompts us to accept truths that can not be proven. You can have faith that God grants you grace, but you can't really prove it can you? I've never been siting in my room and gone "Oh boy, I just received some grace just now!" We accept, on faith, that at moments in our lives, God is giving us the grace we need to do what must be done. We can't measure it or study it in a lab, so we can't accept it on, say, reason or logic alone. Some things we don't really need faith for. I don't need faith to tell me that if I leap from a 20 story building I will fall, probably to a horrible death. The theory of gravity and the knowledge of my body's fragility (Reason) tells me that.
Luckily, we humans have the privilege of operating with Faith and Reason. yay us!