For larger organizations that can afford to follow the sun: the Google standard is "2 teams of 6-8 people separated by 5+ hours time difference, a pri/sec from each geographic team oncall for 12h x 7d at a time, OR 1 team of 8-10 people, oncall for 24h x 7d at a time" https://twitter.com/mipsytipsy/status/1309602522592468992

In either case, you preserve the property that you are either primary or secondary oncall approximately 20-40% of the time, ensuring you keep fresh on your ops stuff, yet have 50%+ of your time to do project work.

Importantly: nobody is expected to ops work when not oncall.

and nobody is expected to project work while oncall; if you are oncall, you are either responding to pages and tickets (toil), or you are doing a "interrupt reduction mini-project" to improve oncall/decrease toil. https://www.usenix.org/system/files/login/articles/login_winter16_11_beyer.pdf