She's sung it 11 times at the Met (6 in 2000, 4 in 2009 and once so far in 2010) and four times with the Met in Japan (2001), and it's highly likely that she's sung it elsewhere in between.

You can check the Met database at http://archives.metoperafamily.org/archives/frame.htm . Choose "Multi-field search" from the menu on the left, then put Fleming into the Personal Names field, Rosenkavalier into the Titles field and press the Search button.