The British Trust for Ornithology has a BlueTit Diary page which sets out their nesting and breeding habits - https://www.bto.org/understanding-bir... - 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. This female took between 6 and 7 weeks! Such a perfectionist. It might go some way to explain why this nest was so deep!