As in most things, it depends. If the soil is sandy or if it's heavy clay it might alter things a bit if you're just planting it for the first time. If you're going to incorporate it into the soil 3 months would probably be the minimum If you're just going to spread it, probably more like 6 months. The clay vs sandy would dictate how much you put on at any one time. In sandy soild it's going to percolate through the sand faster to a depth that young alfalfa plants can;t redily use. As the root grows deeper, the longest I ever saw was around 7 feet long, the plant can use the deeper lime. That's why I would apply smaller amounts several times rather then a large amount all at once.
dun