Use %e format for effort. If it gives you an error you need newer
org-mode, as the patch to fix %e format only got applied recently.

At Wed, 17 Aug 2011 15:14:12 -0700,
Michael Gilbert wrote:
> 
> Hi all â€> 
> I want to be able to skim across my agenda and see effort estimates. I have been looking at the variable org-agenda-prefix-format and wondering if use of the expression evaluation option in there is the way to do it. However, while I have configured this prefix to suit me in other ways, writing a suitable expression to include the effort of the item is beyond my current skills. Any advice?
> 
> TIA!
> 
> — Michael
> 
> 
>