How hard do I have to fall for this thing to activate?

It will be the kind of fall that leaves visible injury, possible unconsciousness, that would need third party assistance.

For a lesser fall you can always ask Siri to call one pf your contacts if you need help

"Hey Siri call Sara Jane Mobile" "Hey Siri call 911". - Use Siri on your Apple Watch - Apple Support

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I have twice had it activated when I jumped off a table after changing a light bulb (or something like that), and that's probably 3 or 4 g's, which would be really a lot if I had landed on my head. I think you have to not only fall, but also land pretty hard. Like LD150 said, the idea is to activate if you are no longer able to call 911 yourself. Luckily, that's not been a problem so far for either of us!

All I can say is harder than that.

The description says a hard fall followed by no movement.

Richard's experience is indicative. As someone too old to jump off a table I haven't experienced the trigger.

Even when I did fall while jogging and smashed my glasses and cracked a rib I immediately picked myself up which might have prevented the trigger. It was after the fall that I bought the Watch.

I would not want to mislead you

should be

"Hey Siri call Sara Jane Mobile"

or

"Hey Siri DIAL 911".

Make phone calls on Apple Watch – Apple Support (UK)