Can I leave broody Orpington in empty nest box?

no problem with doing that but she may stay broody for several weeks. Make sure you do remove the eggs frequently though during the day and also make sure the others can still get in to lay.

Our Orpie went broody she didn't care that she was sitting on zero eggs lol! We took advantage of this and bought some fertile eggs  Of course this is something that you might not want to do 

Aww yes I know what you mean. I only have 2 hens one Bluebell and one Orpie, she is also broody. She is one of last years' offspring.

You could get an auto sex breed of hen such as the Cream leg bar which enables you to determine the sexes directly after hatching by their markings rather than having to wait weeks and weeks. This is what I did. I appreciate though it's not everyones choice to do things this way as it means either facing rehoming/disposing the Cocks (if you get any!!) or disposing of day old Cock chicks. Like I say no everyones cup of tea. x

My orpie cross is also broody .  I take her off the nest at least once a day so she can have a drink and something to eat  (hopefully) and then she just goes and sits back in there again.  I always take the egg away.  There were 3 in there today so the others must have snuck in while she was having a drink.  Generally over the past few weeks the others have been laying around the garden!!
My broody even had a dustbath today so she must be coming to the end of her broodiness? 
Is it true they are only broody as long as it would take to hatch an egg?

I've got a feeling they are broody for 3 weeks but is that how long it takes to hatch an egg?

More like 5 weeks......depends when you take it from. I count from the last egg laid.
My Speckledie delights in being broody maybe because as the bottom of the pecking order she has more kudos!
She starts by chucking straw and twigs over her back and being noisier than usual but still lays. After a week or two she stops laying and gets nesting then when she plucks her chest feathers to line the nest, won't leave the nest box, makes angry chirrup noises if disturbed and runs round like a neurotic headless chicken (!) if locked out of the nestbox I know it's full blown broodiness and I won't see an egg from her for weeks.
The others seem to lay better to fill the nest up for her.......aaaah!