UK Charter Airline.
We have either individual SBY usually for 6hrs, does vary sometimes. These are protected by our union agreement and restrict the duties that you can be called for. We also have a monthly limited number of Block SBY 2-5 days where you are less protected from tours. Occasionally, we have Contactable for 2 hours, where you will be advised of a future duty or roster change.
Your liability to be called varies by base, type, seat and time of year. Personally, I get called about twice a year.