I think the birth cycle of the spider mites is 4-5 day (if anyone cane chime in that would be great) but you want to make sure you kill them AS SOON as they hatch, otherwise they will lay more eggs before you kill them. if the get to lay more than a few generations they will atart to build immunitys to the mitecide! I would go for a more offensive approach, spray every other day for 6 days to get rid of any mites that hatched from eggs that might have already been layed before you sprayed, or else you'll have a week of eggs hatching and maturing and laying more eggs... then after 6 days go one a week, for a couple weeks, u should be mite free!