Here's a little montage which takes us from an empty nest box to first egg. The British Trust for Ornithology has a BlueTit Diary page which sets out their nesting and breeding habits - https://www.bto.org/understanding-birds/articles/blue-tit-diary - and states that usually blue tits spend between one and two weeks building their nest. Some even build their nest in only a few days. Our female took between 6 and 7 weeks! Such a perfectionist. It might go some way to explain why this nest was so deep!

BTW As of the 7th June the parents have been spotted still feeding the fledged chicks in the garden.