That will really depend on a number of things. Just because you are not measuring any PO4 does not mean that there is not any available for the algae. The algae itself may be consuming it faster than the test kits can register it, and it may be coming from bound nutrients in your rock or substrate. If you are not manually harvesting the algae, it may still take months to try and starve it out.
Then again it may react very quickly to its nutrient source being depleted. There are too many factors to really say for sure. I would expect that you would start to see at least a little difference in rate of growth of the algae or even reduction of amount in a couple of weeks to a month of when you get nutrients to zero.
One word of caution. Do not lower your PO4 too quickly. Aggressive use of GFO can cause issues with alk stripping and the sudden change in water quality can cause issues with some corals.
