How do you decide when it's time to buy new tires?

It’s time when you start to think it may be time. Don’t wait for the wear bars. It’s not your opinion, it is too late and you have already been driving unsafely for some time.
Also, just because there is good tread does not mean they are always ok. If they’re old and you can see visible dry rot or cracking of any kind, it’s time also.
I usually wait until the tread wears down to a point where rotating the tires will not make much difference anymore. There is usually a safe amount of tread left still. I also don’t like the “long-life” tires, the kind you can get 50k+ miles out of. IMO they’re not sticky enough and not as safe as one that has a softer compound and grips the pavement better. Just because a tire wears out faster does not mean it’s a bad tire. It’s often the exact opposite.

I bring my car regularly to the dealer for oil changes, and also to Costco for free rotation, and either one of them will let me know it’s time to get new tires.

There is the “penny test”. Put a penny in the tread, Lincoln’s head first. If the tread does not touch the top of Lincoln’s head, your tires are too worn.

I have a mechanic I trust. I’ll replace them when he tells me.

What was the tread life rating on the current tires? Do they seem like they have excessive wear on the thread or outer treads where you corner? 45K is about the time to start thinking unless they are high thread life 60k or over.
If you think you may need to do so soon, and are in a winter weather area, then do the deed for peace of mind. You then get a reset for several years. Nothing worse than hydroplaning or sliding on worn tires.
I changed out my tires after winter season when I took off the winter Mud Tires on my Jeep. New AT mid range tires cost me $1100, but I was happy to know my Jeep’s shoes are new.

If you live in an area with ice and snow, this Fall would be the time to get All Season tires.

For better or worse, I live in Georgia, and snow/ice/wintry weather is a rarity.

Rain, we get a lot of, but fortunately snow isn’t much of an issue.

I literally had new tires installed yesterday. I bought them, because at my last annual inspection in May, the man said he wouldn’t pass those tires next year. I used them until there was nothing left.

I replaced them with summer tires. It’s one of the nice things about living here.

@Hawaii_Jake good point about inspections – mine is due in October, so this may be a good time for me to deal with it.

I also do the “penny” test that @zenvelo described.

The tread wear indicators cast into the tires aren’t there to make noise, they’re just to alert you (and your friendly tire service person), visually, when it’s time to replace them.
When at least three of the wear indicators are level in wear with the rest of the tread, it’s time to replace them.
All most all radial tires are “all season.”
When in Korea during the Korean War, the US Army discovered that it isn’t the tread on the tires that give it traction, but how much rubber is in contact with the road. So it’s just as important to keep the tires air up to the proper pressure, it’s also important to not OVER inflate the tires.
Always inflate with tires cold (driven less than one mille) and follow the vehicle’s recommended tire pressure, NOT what it says on the tire! And check the pressure once a month with a good gauge.

We are told here where I live that tires should not be used for over four years due to heat. Well, I haven’t bought into that due to the fact that such recommendation doesn’t seem to be given in other places that are just as hot as it is here. In the past (before I moved down here) I always had a regular car guy that took care of the car. When he said I needed tires I listened. He didn’t sell tires. I have six recommendations for the guy I’m now using. I will wait to see when he recommends tires. My car is a 2017 with original tires that look new to my amateur eyes.

Penny test here, too. In the ice and snow we must have good tread.

@SnipSnip Did you know tires have an expiration date? Every tire has a birth date—the day it was manufactured—and an expiration date that is six years from that manufacture date.

If your car is a 2017 with original tires, they are past the expiration date. A mechanic will not repair or rotate them, and you should replace them as soon as you can.

If it’s getting close to the wear bars, it’s time to look for sales on appropriate tires. That way you can save a few $. Get them before the rains come. That’s when you will really need the treads.

I use a set of digital calipers with a depth gauge that I purchased at Harbor Freight for about $12. (I see they are advertised on line for $24. I can assure you I did not pay that much.)
When I get new tires I measure the tread depth to the bottom and call that depth 100%. I also measure the tread depth to the wear bars and call that 0%. Since I’m “retired” (see what I did there?) I only drive about 5000 miles per year so I only check my tire depth about once every 1 or 2 years. With those 2 numbers it is simple math to see what % life they have left. The process also quickly shows if one tire is wearing more than the others signalling that I might need an alignment.
I hate wasting money so I try to only replace tires when they are down to 0% – touching the wear bar.

That’s a bit dangerous on a rainy day. That was one contributing factor when I crashed my Prius: rain + low tread.

@zenvelo In all the years I serviced tires I never had a problem with an “old” tire being on a vehicle. An expiration date is just the industry’s attempt to get you to buy new tires, the same as expiration dates on food. What a scam!

When the tyre man says so.

I look if the tread is worn. Either worn low or worn unevenly. If I rarely use the car and the tires are many many years old, I would probably consider changing the tires to be safe even if the tread wasn’t that bad.