It can last up to 3 months depending on temps, if you added mlf nutrient, and what the specifics of your wine was like as in PH and ta cause these factors can slow down MLF just like they can throw off a standard fermentation. You can find out if its done by doing a chromatagraphy test.
It works very simply by drawing out a small wine sample into a plastic sqeeze tube. The sample is applied to a test strip and after 4 minutes the color is compared to a color chart. Much like pH strips, but none of the problems inherent with reading pH strips because the colors are completely different than wine color.