I'm not aware of any time frame, though I guess there is probably info on the company website.
Assuming its being applied properly, its highly unusual not to see a rapid improvement. I've rarely used VAC for more than 2 weeks, it does its job within that timeframe.
I would want to check the wound for infection, (swab) and investigate the local perfusion. If its a venous ulcer, most likely compression bandaging would be a more effective therapy, but its difficult to know without seeing the wound and having more history.