Too many variables.
If you only drove 100 miles a year and you kept the car in a garage then the answer is likely to be a long time. However, take it to a track day and see a set of tyres disappear in smoke in a day or less depending on, again, many factors, inc' front wheel drive "v" rear wheel drive, agressive nature of the driving etc.
Your tyres have a tyre wear indicator throughout the tread tyre, it's known as the bridge. There are legal requirements as to how deep tyres in the UK must be and across what % of the tyre. Is this what you are asking?
If in doubt call in at a garage or Google for the legal bottom line.
Inspect your tyres for wear and tear at least once a month I'd suggest and beware extra bulges in the sidewall, especially on the front tyres.
If in doubt - have them replaced.
New tires can cost anywhere from $80 to over 300 for an average car.
20-30 lbs
2850
10 to 20 dollars
So the tires will "wear" evenly and you will have longer "tread-life"
The average tire is around 40lbs(family sedan tire) of that would be around 55 tires.
Depending on the quality of the tires, they can be sold from anywhere from a couple hundred dollars to a couple thousand. For the average car, they will be relatively inexpensive.
depends on how much air in tires
Yes. The tires do have an expiration date. It is a must to check when you plan to change tires of your car, otherwise, you are placing bets on your life.
Car tires vary greatly based on rim size. Smaller tires are cheaper than large tires. Firestone tires for a mid size sedan range from about $60 to $100.
I would NOT recommend buying tires online ! -You want to physically see what is going on your car. Your life will depend on it.
There are so many different sizes there really is no average. Sizes are changing all the time.