Interpreting an SSID closes to me, with 0 activity (noone connects there). According to wireshark, beacon takes 516µs at 6Mbit/s with 40 beacons/s, which equals 500e-6 * 40 = 0.02 = 2% airtime goes to a single SSID with 0 traffic. Typical settings are 10 beacon/s which would be 0.5%, but 2.4Ghz has 1Mbit/s default rate, so about 3%.

With 20 idle SSIDs on 2.4Ghz with default settings, roughly 50% of airtime is spent on SSID broadcast.