I sent query letters to probably 40 or 50 different agents over a period of almost three years and was rejected every time. While most of them seemed to enjoy my work, none of the 3 or 4 who seemed the most interested had the time "to take on a new, unknown author" - their words, not mine. My solution was to self-publish; however, at this time my writing is more of a hobby. From other people I've talked to and things I've read, it would seem that it takes, on average, about five years and many rejections before you get accepted. Don't give up if you really want to go the traditional route.