Topping the plant is meant to make the side branches grow longer/taller. That means the plant stops growing upward for a while...about how long does it take to get it growing upward again? If I'm wrong about topping please correct me.
Sort of both. Everytime you top all of the tallest stalks (the first time all of them is one, of course), the two small branches on either side of each stalk and directly under the cuts will start growing faster. They grow the fastest because they become the closest branches to the light after topping. This causes the stalk to split into a V from that point upward, in addition to providing a growth spurt to lower, existing branches. In the time it takes for each stalk to recover from being cut & get the two branches growing, the other stalks originating from lower on the plant will grow taller. If you top all of the stalks at canopy level enough times, eventually any lower branch/stalk getting enough light will reach canopy level. How long this can take depends on how much light each branch is getting and how well that particular strain branches naturally.
Under good conditions, the stalk you top should show stronger branch growth 2-3 days later. Always wait at least a week after topping to force flowering.
Is right before flowering the best tip to top. I know u get that stretch from some strains when u switch but will it benefit topping in veg?
There really isn't a tried-and-true "best time" to top, especially since a grower can trim & train a plant to his or her heart's desire before initiating flowering. The last cut often comes a week before flowering. Not sooner because the plant needs time to heal & start its new growth pattern. Flowering too soon after topping can result in smaller colas than would have been produced otherwise. Not much later than a week because the reason that the last cut was the last cut is the plant finally achieved the size & shape the grower was training for. Letting it grow too much longer would cause it to grow out of this desired shape, perhaps no longer fitting in the grow area through to harvest. Once you switch to 12/12, pruning stops. Once in awhile trimming off a leaf that's shading a budsite is fine, but for the most part you want to avoid cutting during flowering.