Sorry -- didn't mean to leave you hanging all this time! Or to leave the lamb marinating this long.
In terms of safety, you could probably leave it for a couple of days as long it you keep it cold. For attractiveness, probably not much more than a day, because the acid will start to "cook" the meat (the same as with seafood) and turn it a very unappetizing shade of gray. And for effectiveness -- again, a day or so. The marinade won't really penetrate all the way through the meat no matter how long you leave it; you won't really be harming the meat, but you won't be helping it, either.
I once left chicken pieces marinating for a few days on the assumption that longer was better. Wrong!

Eventually, the flesh started to break down, and cooked up to mush.