If it is squirted on through the installed rain repellent system, then under no conditions do you do it on a dry windscreen or light rain. If however, the rain is so heavy that you cannot see the runway properly through the windscreen then that is the time for one squirt only. The wipers should be on by then. It is effective immediately.
Boeing used to recommend one windscreen at a time. If squirted on a dry windscreen it leaves a coating. Our engineers discovered that shaking a bottle of Coca Cola and squirting that all over the windscreen successfully cleaned away the residue
