Some people use 2s as an aide-memoir, so I am curious as to whether the majority on here see this as being useful...
You can last 3 minutes without air, a resting adult breath cycle takes at least 3 seconds.
You should consider shelter and clothing after 3 hours of being in an open environment.
You can last 3 days without water, allow 3 litres per day to stay healthy when exerting yourself.
You can go 3 weeks without food, but only if you have water and shelter and are not exerting yourself.