How do you know when to pull up onions?

No where near ready! Did you start with seeds? Sets? Or transplants?

https://harvesttotable.com/onion_growing_tips/

I grow a lot of white onions just from the roots of a previously harvested onion. I can grow year round. Depending on the season it can be anywhere from 3 months to 6 months. I just leave in ground or in pots and pull as I use them. I also let some "go to seed" to start more plants.

when the end start turning brown and fall over its time to pull them up

Depends. I love the small green onions, so you can pull up and use as needed. It will also thin any crowded ones. Great for stir fry or salads. Great thing about onions is that you can use them at any stage!

Hi Karen, Leave until the leaves fall over and start to die back. They will be fine left in the ground for some time yet..........

1bend the green tops over, close to the ground. Leave then that way til fall. The greens will die off but the bulbs will continue to fill out. They actually like a bit of frost to sweeten them a bit. I always leave mine in the ground til mid September. If you pull them earlier and store in someplace warm, they will sprout and rot. cut most of root off and let the skins get good and dry before storage. Remove any remaining tops. With luck they'll just pull off,

The seed packet or garden center can tell you how long your variety takes to be mature. if They seem to be getting larger than you want, you can harvest early. Keep them cool and in a dark place and they should hold, thiugh sweet varieties do not store as well.