What time is reasonable?

[upinclouds]

To carry on partying in the garden?

Neighbours have been having a party for their young adult son since 3pm. They've (family plus loads of teens) all been out in the garden drinking and shouting at the top of their voices since then...no sign of it winding down 🙄

If it’s a one off, I’d hope they would start winding down between 11 and 12.

Surely 11pm on a Sunday. Midnight on a Friday or Saturday

11pm latest

For a Sunday I'd agree with 11pm.

Gosh you're more tolerant than me, I'd be pretty pissed off come 10pm on a Sunday, especially if it had been going all afternoon already.

I presume its not a party every weekend?

If not, live and let live. Its a birthday party. People are allowed to celebrate and it wasn't that late when you posted.