As often as they require replacement. If the trailer or drive tires get to 2/32nds, replace them. Steers have to be at least 4/32nds.
In the average truck that does not haul large loads the tires should be replaced every 40 thousand miles. This is the best way to ensure that your tires are giving you the best gas mileage.
It depends on how old your tires are and how warn they are. The front tires will wear out first, so if they replace your tires regularly then you can get away with only replacing the front two. If you rotate your tires when you change your oil or don't replace your tires often enough then you will probably want to replace all four at the same time.
not too often unless they dont work
As long as all 4 match, the height difference between 65 and 70 should not cause a problem on most vehicles.
When your tires have 2/32" tread depth left you need to replace them.
P215/70R15 are alot tires. They will probably rub on most vehicles that call for P215/60R15. Most tire dealers should have the specs on your particular vehicle.
The 185-60-14 tires are general tires. They can fit most vehicles and are all season tires.
On average you should rotate your tires every 6000 mile or 6 months.
When your tire treds have gone bald it's time to replace ATV tires. You can also have a qualified dealer look at the ATV. The rules are similar to the tires on your car. You should replace ATV tires when you start to see the tread worn all the way off the tires. You should also replace tires that are showing wear of the rubber itself - cracks, bulges, flaking, etc. as these problems could lead to a catastrophic tire failure during use.
Airless tires are tires which are not pumped up with air in the traditional way. Vehicles such as ride-on lawnmowers and golf buggies use airless tires.
You should check your tires regularly. If your tires are properly inflated with no leaks your tire pressure should not drop much over time. Some people wait until oil changes to have their pressure checked, and this is usually often enough. Others check every time they get fuel.