Typically, advertisers have 30 days validations period (to meet standard cancellation/returns policies). After this time has elapsed, the advertiser can no longer cancel the transaction and it will be invoiced and confirmed.
In some cases, such as with travel advertisers, validations period will be longer, perhaps working on a "second-level" system (involving transactions tracking at initially £0.01 and later upgraded). For further guidance on second levels, view here.
If you are ever unsure about an advertiser's validations policy, please get in touch.