If sitting is the new smoking, is a standing desk the equivalent of giving up smoking? Maybe. Or maybe not. At the moment, we don't have strong evidence to say standing for longer produces health benefits, never mind the standing desk craze.
The Cochrane Library reviewed 20 studies of "interventions aimed at reducing sitting time at work." Sit-stand desks reduced sitting by between 30 minutes and two hours a day, depending on when the measurement was taken (soon after installing the desk, or later). But the studies tended to be small (across all 20, only 2,174 people participated) and, in some cases, badly designed, and only measured workers for a short duration.