How long between building nest and laying?

[werdnal]

We have Great Tits who have been actively building a nest in our camera bird box for about a month. There was frantic activity leading up to Easter, with loads of moss and hair from our pony being carefully added to it and we were anticipating they might start laying any day, especially as the weather has been so spring-like. However, things have now slowed right down. One of the pair is still roosting in the box every night, but there is hardly any activity at all, and still no eggs.

We are hoping that both of the pair are still here, and that they haven't lost interest and started another nest elsewhere. Anyone know when eggs might appear?