Hello, I'm interested in starting to practice speedrunning Portal 2, but I'm not sure when exactly I need to start the timer, i.e. as soon as I can move in the bedroom, as soon as the first portal opens, etc.

If you use the autosplitter with IGT and SAR (available in the resources) then it should automatically start. This is the easiest and most common timing method, as otherwise your time will include loads. For this, you can load the vault skip save and start at 5:16.33 seconds. For starting at the actual start of the game (upon waking up for the first time/starting a new game), timing starts at 0 seconds when the crosshair is visible.