The answer to this question is dependent on several variables. For example: What is the starting sugar content of your sap? What type of pans/arch are you using? What type of firewood are you using? How often do you recharge the firebox with more wood? And so on. Due to all these variables, there is not a “one-size-fits all” time frame.
That being said — if you are using a continuous flow pan — expect to boil for several hours before your first draw-off. After your pan has been sweetened, draw offs will be much more frequent. Watch your syrup temperature and/or take periodic hydrometer readings to monitor the progress of your syrup. With every moment that steam is rising up off of your evaporator, you are getting closer to finished syrup.
If you are using a flat pan, you must wait for the entire pan to become maple syrup. So you will not be able to draw off syrup until the very end.
For more information, see the Jones Rule of 86.