i'd say as much as can be easily bent w/o snapping. this method (lsting) can be used to change where the biggest/best growth is. the top bud is (under nominal circumstances) is the best. when you bend it down , the next highest shoot will take over. i've seen plants grown longways along the ground with lots of buds coming straight up off of them.
just be sure to use a thicker string (not fishing line) and a slip knot so that it won't cut the plant as it grows. and search the grow journals for some lsting...