How long should it take to delete everything on my iPhone 4?

For a little clarification, I have about 5 - 5 1/2 gigs taken up, and I just want to know how long resetting everything should take.So, how long?

My iphone was completely full. (8 gb) and I reset it to erase everything on my phone. It took no time at all, now that I was really paying attention to begin with but I recall it took less than 10 minutes to have my phone restored to brand new