Tires usually lose about 1 psi a month. If it is a low profile tire, it can lose more than that. Also, tires lose about 1 psi for every 10 degree drop in temperature (Fahrenheit). So, in a year, each tire may lose about 12 psi. That's not even taking into account the amount it changes from temperature.
With modern tires it can be very difficult to tell from just looking if a tire is low. Unless it is truly a "flat tire," an underinflated tire will often still look ok.
It's probably a good idea to check tire pressures at least once a month. That way you can add some air if needed. Also, if there is a leak from a nail or something, you will catch it early. Because when a tire is too low in pressure, it heats up a lot when driven on (due to needing to flex a lot more). This makes it prone to blowouts or other failure that requires replacement. If you catch a leak or nail early, it can usually be repaired for a lot cheaper than tire replacement.
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