The short answer, every 60-120 seconds. The more detailed answer requires a bit of explanation. 

AirTags work by relaying their location to other compatible devices nearby on Apple’s “Find My” network.  More technically, AirTags work by broadcasting an encrypted Bluetooth signal that is detectable by other nearby and supported Bluetooth devices. 

Devices that are on the Find My network and which are connected to the internet can and will passively report the location of nearby AirTags to iCloud.  Once an AirTag associated with your AppleID is detected on the Find My network, it will be displayed on your Find My app.

Assuming your AirTag is within Bluetooth range of one or more Find My network devices that can report its location to iCloud, it will update its location every 60-120 seconds. 

Depending on the density of other Find My devices nearby, this location data will range from approximate to very accurate. 

If no other Find My network devices are nearby to report the AirTag’s location, then it will stop reporting its current location entirely until a Find My device with an internet connection gets close enough to relay the AirTag’s location.

Related Article: How Can I Find My Apple Airpods Case?