The right cover-off time speeds up dissipation and stops the smell building back up. Here are the windows for each shock type.
Cover-off time after a shock is doing one specific job: letting the gases vent. Chlorine and oxidiser by-products are volatile, and trapping them under the cover defeats half the point of shocking. The right window is shorter than most owners think.
Non-chlorine shock: 15 to 30 minutes is the productive window. After about 30 minutes, the shock has done what it is going to do.
Chlorine shock at maintenance dose (5 ppm): 30 to 60 minutes.
Chlorine shock at high dose (10 ppm or more): 1 to 2 hours, or until free chlorine has dropped back below 5 ppm. Test before covering.
For all of the above, jets should be running on slow throughout.
If it is raining or below 10C, lift the cover slightly with a pool noodle or a wedge rather than removing it entirely. The gap of a few centimetres lets the gases vent without losing the heat.
If it is windy, a lifted cover can also act as a sail. A noodle wedged into the corner is more practical than fully removing it.
In snow, a fully removed cover is asking for trouble. The brief venting is not worth a tub full of melted snow and a damp cover. Use the gap-vent method instead.
Routine maintenance dosing of free chlorine or bromine top-ups: cover stays on. There are no significant gases to vent.
pH adjusters: cover stays on, jets running for 30 minutes.
Calcium increaser: cover off for 15 minutes so you can watch it dissolve and brush any deposits off the floor.
The shock still worked but the chloramines vented later when you opened it. Smell intense but harmless. Run the jets with the cover off for 15 minutes the next morning to clear it.
Yes, more so. Indoor installations need active ventilation during shocking. Crack a window or run an extractor fan. The gases do not magically dissipate inside.