Every 12 years.
Commercially speaking, in the UK they have to be checked annually, and signed.
A date is stamped on the neck/bottom of the pressurized dry chemical extinguisher. i.e 04 - could mean 2004 or 1994, but nonetheless, the 4th year. It could be on a label, but should be...
Once a week.
I believe fire extinguishers are inspected yearly.