This is a difficult question because it really depends on the project. It also depends what your role is. If you're the LEED coordinator for the project you could spend, say 120 hours. If you're the architect (ie. compiling most of the documentation) it could be twice that. The truth is I'm really not sure. I assume you boss is just trying to budget your time, or the budget, however without introducing her to the process this will be difficult. 
Perhaps she'd be willing to take a look at LEEDuser? She could go through the credits in the system you'll be using, that would give her a good idea, without getting her all set up on LEED-Online.