There are no rules about frequency – you can aerate just once a year or up to 4 times if necessary. You can even aerate monthly during the correct months if you use a pencil-tine with its smaller diameter which causes less surface disruption – but follow up with a top dressing each time.
HOLLOW TINE AERATION: Given the spacing between the tines, the machine or tool will typically aerate approximately only 5% of the overall lawn area, leaving plenty of soil untouched. So, while once a year is the minimum, twice is better.
SOLID TINE AERATION: This should only be done if you have very good soils, although on any soil it will improve surface drainage to some extent. If this operation is for you, then you can do this as often as conditions will allow.
SLITTING: If using slitting, this should be done frequently to be effective; weekly or fortnightly during winter is generally sufficient.