That depends on the strain of plants and several other environmental factors. But generally if you veg growth is healthy and vigorous then the flowering growth will be about the same. You will typically see them shoot up towards the light a bit faster than when in veg. This is because the change in lighting schedule causes them to stretch some until they switch fully to flowering.
without a doubt genetics plays a part. generally speaking sativas stretch more than indicas. the light can contribute to the stretch as well. 2x is a good rule of thumb, but I grow plants that stretch 3x, 4x, and even 5x in flower. So a 7" plants will be 4' when done.