The Metropolitan Opera Presents (PBS, 8 p.m.- 1 a.m.): No, those hours are not typos. The opera is `Parsifal,' a Wagnerian marathon if there ever was one, full of what its many nonfans view as murky music that goes on more or less indefinitely. But in fact it offers some of Wagner's most evocative and haunting music, and has proven a transcendent experience to generations of audiences. Wagner's final opera, it is set in Spain and based on the legend of the Holy Grail: A `guileless fool' named Parsifal le arns of the loss of the holy spear and eventually returns it to the Knights of the Grail. With its theme of redemption, the work is traditionally staged at Easter time, although whether it is sacred or profane has been in dispute ever since its first production in 1882. The PBS version, sung in German with English subtitles, was initially done at the Met in 1991, then taped in 1992 during spring performances. The staging is by the same team under Otto Schenk that mounted the `Ring' cycle seen on PBS over fo ur consecutive nights in 1990. It features Siegfried Jerusalem in the title role, Waltrud Myer as Kundry, Bernd Weikl as Amfortas, and Kurt Moll as the knight Gurnemanz - all under the baton of James Levine. Roadhog (MTV, 10-10:30 p.m.): TV has been doing all right by bikers lately. Not long ago, a show was devoted to the Harley Davidson and its reverberations in American culture. Now motorcyclists are being spotlighted again in a documentary look at their music, fashion, lingo, and lifestyles. Some of the biker types may surprise you. One group featured is the `Lynch Mob,' male bikers employed by the brokerage firm Merrill Lynch, and there's a Christian ministry group. But the show does take a look at the Hell 's Angels, and it also interviews several celebrity bikers, including motorcycle fan Arnold Schwarzenegger. 48 Hours (CBS, 10-11 p.m.): This edition of the weekly news feature, `Not in My Army,' examines the issue of homosexuals in the armed forces - just as the US Senate Armed Services Committee is holding hearings on the issue.Please check local listings for all programs, especially those on PBS.