How to know if my budgies are interested in breeding?

2 of my budgies are a bonded pair. They preen each other, regurgitate,and play with each other. I got them from different places so they are definably not related. I've done a lot of research over the 2 or 3 months that it took for them to get bonded. I've added a nest box, a mineral block, they have a cuttlebone, they are on a pelleted and seed diet. And yes I know they need fruits and veggies. I also removed the grate from the bottom. I put the nesting box in yesterday and today I noticed the male looking at it. Then he started to get excited and was chirping and going in circles. Soon after that I noticed the female just peeking her head inside. Is she interested? That is all she does is poke her head inside and how long do you think it might be till she gets enough courage to go inside? They also have a light hanging over them and i was misting water over the cage to make them think it was raining. Anything i need to know? And any other signs I should look for to see that they are interested? Thanx!