In my (primary) school, they happen yearly-done by the SMT-graded according to Ofsted criteria, with a week's notice, ie we're coming to see you teach maths next Wednesday at 10am. We also have other more informal peer observations for subjects.
My sister in law's school does observations by two members of the SMT at once-they are with a weekend notice but are unannounced-you are just told it will be sometime during the week. This happens once a term and if they are considered requires improvement, they are revisited within a fortnight with it counting towards their PMR.
Can I be nosy and ask what happens in your school?