Here's your answer:
https://www.ford-trucks.com/forums/9...ml#post8337067

Also look at this:
https://www.ford-trucks.com/forums/9...ml#post8219650

Additional Info:
https://www.ford-trucks.com/forums/7...ose-plugs.html

If you search "spark plug" in the '2004 - 2008 F-150' forum you will find a lot of answers to your questions.

So far it looks that maybe leaving the plugs in the head longer will run the chance of them becoming weaker but others say leave them in till they are due. My advice would be to follow the advice of the three posts above and take your time.