Koji + Mash -- Recommendations Please

> From what I gather, mash is the tool of choice for composing usable 
repositories out of koji's build efforts.  What advice do y'all have for 
integrating the two?  Presently, I've got mash running via cron every 15m 
but that hardly seems optimal as it's a lot of busy work if I'm not 
building packages for several days and conversely adds more latency into 
my build + deploy + test timeline if a build finishes right after mash 
does.  Seems like a hook or callback is in order, but I've not seen any 
write ups on this subject.  Also, I had a rpm built by koji which then 
made it over to my repo via mash, but then I got busy for a few days on 
other tasks.  When I came back it appears that my rpm has disappeared from 
the repo.  I'm guessing kojira cleaned it up, but I was under the 
impression that such cleanup wouldn't propagate to the repo.

--
John Florian