well, it all depends. some people think learning sax is easier to learn than clarinet/easier to learn sax then clarinet.
but if you have a clarinet to use, and you aquire a few technical books and instructional books, you may be able to learn enough to play in a marching band with in a year. remember you have the dissadvantage of not having years of expirience on clarinet, and if the band your looking to be in plays higher level music, it may take you more years than that. although, most of us clarinetists started from nothing, but u have a strong piano backround already have the music fundamentals down, all you need are fingerings and tone quality. the best way to go would be a private instructor. i say go for it, you can do it.