it depends...
first off what type of insecticide are you using?
if you plan on using an oil based product, like neem or oganicide, you nwould want to apply the treatment first thing in the morn while the sun is coming up. make sure to spary the undersides of the leaves beacuse this IS WHERE THE MITES ARE HANGING. Re apply every three or four days till you see no more mites. the next question.... once again depends on the insecticide you use, but you may spray
neem oil at any time during the plants life. i would reccomend to stop treatment if your "bloom" cycle is past 25 to 30 days. i really try NOT to spray anything on my plants during bloom. (personal preference).
NOTE: on some insecticides, the label that states" for use up to day of harvest"
PLEASE REMEMBER THAT THE MANUFACTURE OF THE INSECTICIDE NEVER MEANT FOR THAT TO BE USED ON CANNABIS.DO NOT USE UP TILL THE DAY OF HARVEST BEACUSE YOU WILL NOT WASH OFF YOUR NICE BUD...but, you can wash that stuff of your veggies though.
prevention works the best.... start an "IPM"( intergerated pest management) program on your vegging plants. please remember that bugs CAN build resistance to insecticides so you should have a few different modes of killing action so the future generations of bugs will not build up resistance. for example.... if you are using neem this week, next week go for "
spinosad" and if then maybe some "zero telerance" the next and the back to the neem and so on.
hope this helps