I have been doing 4-5000 miles or one year for My 2004 F-150 since I bought it new and the extended warranty just expired in may of this year. The manual says 5000 miles for normal and 3000 miles for severe. My use falls under severe, but I still followed the normal schedule. I figured 3000 was too soon and they don't know how I use it. The last few years it has only had about 4000 miles on the oil when I had it changed though. The gas mileage really drops on this truck after 4-5000 miles on the oil. I am planning go the full 5000 miles do a UOA on it the next time I change the oil.
My Focus I have been doing every 5000 miles lately as well. I am trying to decide if I should go longer. The manual calls for the same 5000 normal 3000 severe and I would say it sees more than severe use. Maybe extreme use. It gets driven hard in the summer(close to red line often), idles for 25 min twice per day in the winter on work days, hauls heavy loads, sits in traffic, makes a lot of short trips, and has been neglected in the past. After 5000 miles, The fuel mileage does not change, with Maxlife and Defy on the last two changes it has not lost any oil (not that it lost much before from the tiny valve cover leak) and the engine even looks cleaner under the oil cap. Although I may just keep with 5000 mile intervals on the Focus too because I look forward to doing it as long as it is at least 30 degrees or warmer, and 5000 miles is easy to remember.