Dilema?

Okay so here is my problem: Have been waiting patiently for my hen to go broody and yey today she has finally decided to sit tight in her nest! However today I also booked time off work to go on holiday in 3 weeks for 1 week (not so yey!). So could I wait another week to put some fresh fertile eggs under her so they would be due to hatch the day I get back? will she sit for over 4 weeks? and would she notice if I swapped her eggs?

Have an experienced chicken keeper next door who is willing to look after all my animals so that is not a problem but ideally i would like to be there which is why I thought about waiting another week before putting eggs under her. She has got 3 of her own eggs and 3 of her pals under her at the moment but I have arranged for a friend to give me some of her eggs as and when the time was right.