Hey Azagthoth,
it totally depends on how long you want to keep them in growing stage.
If you like to grow them for like 4-5 weeks total, i would say defoliate a week before going into flowering,
but if you like to grow them for longer, like 7-8 weeks, you should defoliate twice before going into flowering :)
2 weeks after flowering you can start defoliate again if you feel like they need too, even weeks after that you can do it.
I know growers that almost defoliate everyweek in flowering stage until the end, but i personaly don't like to stress them to much and like to keep as many ''solar''leafs for energy.
I only get rid of the leafs that shadow my buds or are beneeth the plant where they no longer can get sunlight. Another reason would be to give my plant some air.
Goodluck with defoliating :)