Bake lightmap with lots of object

Hello,

I am now in trouble with lightmap of Unity. I made my map with a lot of blocks and cells, and now I have over 1500 objects in my map. The bake progress takes me over 8 hours, is that normal? And is there any ways to decrease the number of light map I have to work with (merging these maps or something like that)?

Thank you very much for reading this question, and sorry for my bad English.