Advice on promotion letters bleg

This fall I have been asked to write seven promotion letters. Writing these takes me at least 2 hours. If it’s someone I don’t know it takes me longer because I have to read some of their papers. Earlier this year, I wrote one for a Biology department that took me at least 6 hours. So how many are too many? Should I set a limit? Advice and opinions in the comments would be greatly appreciated.