How long should I press the WPS-button?

The best answer to this is 'it depends'. Allthough you might thing they have created on default setting for this, they did not. And 'it depends' really means that it depends on the manufacturer of you Wi-Fi router. Generally speaking you need to push the WPS-button anywere between a few seconds until a full minute. Mostly until it starts flashing. After this you generally have 2 minutes to connect your device.