apple watch series 6
display goes off in 5-6 seconds when set to always on
Always On is turned on by default on Apple Watch Series 5 and Series 6. In this mode, the time is always visible, along with your watch face or the most recent active app.
To preserve battery life, the display dims when your wrist is down, or by a quick gesture of covering the display with your hand. A wrist raise or tap on the screen brings everything back to full brightness and makes the watch screen active so you can interact with your watch.
For more info, see:
Use the Always On feature with your Apple Watch - Apple Support
Also see fireyjack's reply to:
Apple Watch series 6 not always on - Apple Community