Chicken on strike!

Well, Korma is officially on strike. 
My research tell's me she has gone all broody on me. She spends all her time in the nest box and when i approach her to collect the eggs she makes a 'clock clock clock clock' noise at me and slightly spreads her wings.

How long does this normally last? The other two, Tikka and Massala, are still laying every day but Korma is just sitting on the eggs and not laying.

Should i be worried about this or just be patient and hope she snaps out of it. Or maybe i should get some hatching eggs and get some more knowing i have a mother hen on my hands!