Depends on plane & tactic.
In a Spit I will keep it on even when dogfighting, unless I'm really getting into trouble and need every ounce of performance, the slipper tank doesn't hurt that much anyway. Similar in a I-16 which has such a lousy range that I need every gallon of fuel.
When BnZ or doing some shallow E fighting in planes like Ta 152 or P 51, I generally keep it on as long as possible, but are more quicker to drop it than in Spits.