Many months. According to the cdc summary, mercury half-life as measured in urine is 1-3 months. (http://www.Cdc.Gov/biomonitoring/mercury_biomonitoringsummary.Html) this means more than 99% will be out of the body after 5 half lives (5 to 15 months).
Good question. It depends on percentage of body fat and your renal and hepatic status mostly. Of course the more mercury you take in the more you tend to carry in your system. Different types of fish contain varying amounts of mercury. If the fish is at the top of the food pyrimid (tuna, swordfish for example) there tends to be a higher mercury concentration.