15 minutes should be fine for non-chlorine shock, although i do subscribe to the after soak dosage method myself. in some situations (like the silver ion low chlorine formula) you have to put the shock in before. in that 15 minutes, with the jets running and the cover off, the oxidizer will pretty much be all gassed off before you soak.
if you want to try the after soak dosing leisure time makes a great new product, replenish. it is a blend of chlorine and non-chlorine shock. it seems to me to be the perfect mix to most effectively remove the contaminents that result from a soak in the tub.