Processing rules are applied immediately to the report suite after they are saved. Changes from processing rules should be visible in your report suite within minutes of saving. When testing processing rules, Adobe recommend configuring Real-Time Reports in your test report suite so you can quickly see the results of a processing rule.
Do we know if this logic has changed? It appears that a change in my processing rules has resulted in it updating historic data. Does this seem right? Haven't been able to find anything on the web about this.