When you're all set, fill your pot two thirds full of water, turn on the heater and wait for the water to boil. After the water has come to a boil, the steam box still takes a little while to heat up. Once you get a good flow of steam coming out of the end of the box, you're ready to heat and bend. One quarter inch thick ribs heat up in under 5 minutes. Thicker stock takes longer. Heat has to travel from the surface of the wood to the interior. Rule of thumb is 15 minutes of heating per 1/2 inch of thickness.
Keep in mind that some woods bend better than others. White oak, red oak, ash and poplar all bend well. Straight grain is important as well. If you have rain running out of the face of the board, your wood will most likely break where the grain runs out.
It's really easy to leave on the heat when you're done bending wood. Eventually, the water all evaporates, the pot becomes hot and the plywood starts smoking. Good way to start a fire.