The time it will take for the pressure to build up inside an instant pot will depend on the contents of the pot. Preheat is the time it will take for the pressure to build up inside the pot and for it to start cooking. This will only happen once you have set the cooking time. It will take about 10 seconds and the cooker will enter the preheating cycle and, in a few minutes (mostly 5), the pot will be out of preheating and ready to cook. At this point, the float valve will pop up.
https://instantpot.wpengine.com/faq/