100,000 miles. 4-8 years.
Iridiums come pre-gapped with a plastic cover to keep them from getting squished in shipping. Don't gap them.
Coppers will require a gap tool... it's a little $0.99 coin sold next to the register at the auto parts store. Set the gap to 0.60".
That's the basic idea.
There really shouldn't be much dirt around that area unless you've been mudding or something. The engine can handle a little dust, so don't lose any sleep over it. ALL engine maintenance is easier if you detail the engine bay first. Plenty of that in the detailing forum.