Unfortunately I can't help you but I'm curious to know why do you need this? Why It's important to you?
If you want to do it, you can use GetTickCount in kernel32.dll (win32) or Environment.TickCount in .NET; before and after of loading a layer.
You want to know time of loading without actually loading it? I mean something like estimation?
Well, it depends on your machine I think.
Can you tell me why do you need this? Where will you use it?