A few years ago we put a box up in the 1st week of April and they moved in next day. I thinnk they normally start sorting things out about March.
Don't forget to try and put some bits and pieces out to help them pack the nest with ie moss, dog hair, you can also get nesting material from some bird food suppliers. We got some and I hung it up in its container but pulled manageable size bits in bushes ...the BTs found it and managed to get the base of our nest box covered in double quick time and all the liitle ones survived as well
Marmot Avatar by Birdforum Member - Pavlik