I've heard varying opinions on this, but I'd say anything 45-50 grams is a decent weight, though I prefer mine a bit bigger.
Most of my females weigh well over 50 grams, with some over 65 grams (porkers).
I won't breed a female under 55 grams, but that's just me. I am all too familiar with leos that like to go on hunger strikes while ovulating/producing eggs, and they can lose weight.
55 grams is a decent breeding weight (though some say you can breed smaller than that, I personally don't like to), so that in case they lose any weight, it usually doesn't get them down to a dangerously low amount of weight.