Bluetooth devices typically advertise every 100ms to 1sec. For beacons, this is configurable. The advertising only takes of the order of 1 or 2 milliseconds so that most of time the device is not advertising. This conserves power and is one of the strategies behind Bluetooth LE's low power.

Most devices send up to 37 bytes allowed in Bluetooth 4.x. Bluetooth 5.x can hold up to 255 bytes of data and packets can be chained to allow for larger payloads up to 1650 bytes. These sizes are still small and contribute to the advertising being short and using low power.

Beacon Advertising Cheat Sheet
Bluetooth Low Energy - It starts with Advertising
KBA_BT_0201: Bluetooth advertising data basics
A BLE Advertising Primer
Choosing an Advertising Interval
Testing if a Beacon is Working
One minute to understand BLE advertising data package
Bluetooth 5 Advertising Extensions
Bluetooth low energy Advertising, a beginner's tutorial
Beacon Advertising While Connected
Understanding Bluetooth Advertising Packets
Using Bluetooth Wireless Sensors
On Practical Selective Jamming of Bluetooth Low Energy Advertising
Bluetooth low energy Advertising, a beginner's tutorial
advlib: an open library for wireless advertising packets
A Practical Guide to BLE Throughput
Bluetooth advertising data basics
Bluetooth advertising using manufacturer specific data
Why Bluetooth LE Scanning Doesn’t Always See Devices (the First Time)
Making More Sense of Bluetooth Advertising Scans
Is it Possible to Continuously Scan for Bluetooth Devices on iOS and Android?
Which Beacons Support iBeacon and Eddystone Simultaneously?