The number of times you have to coat your knife will vary with use and environmental conditions but as a general rule Tuf-Cloth works very well to prevent rust. I use it all the time in an area of the country that had nearly 100 inches of rain last year and none of my knives have any rust.
If you're near the ocean you may want to try Marine Tuf-Cloth since it has an extra ingredient that leaves a film on the blade. I live near the ocean but do not feel I have to use the Mrine Tuf-Cloth.
It's a great product which really works. I usually just wipe down my carry blade at the end of the day and I put it away, worry free.