i think a general rule of thumb on any standard transport is 60,000 miles
How to change the spark plugs
It is moderately easy to change the spark plugs on a Liberty, although it is rather tricky because of the gap between the spark plugs.
Sell it !!!
Spark plugs and wires
Changing spark plugs is relatively easy. Disconnect the batter and spark plug cables, and remove the old spark plugs. Next replace the spark plugs with new ones, reconnect the spark plug cables and battery.
Have a look at ; www.carbasics.co.uk
This is for the '05 Frontier, but should be what you're looking for. http://www.clubfrontier.org/forums/f23/how-change-spark-plugs-2005-gen-2-frontier-40507/
Disconnect the battery, remove the spark plug cables and boot, and remove the old spark plugs. Put in the new spark plugs, attach the cables and boots, and connect the battery.
Of course this isn't true. They're just trying to make $$$.
usually every 100000
I just replaced the plugs on my 2005 tundra 4.7 v8. they are easy. their is so much room to work with. But do use the genuine Toyota spark plugs. "Do not use aftermarket plugs" i garauntee if you use the aftermarket plugs, you will wreak havoc. the heat range is too high on the aftermarket plugs, and therefore will burn holes in the ignition wires. check eBay for Toyota plugs or your local Toyota dealer.
NGK platinum plugs