Generally not metered rolls. It's similar to tympani rolls; slower on low notes than high ones. Basically whatever feels right within the song.
Now there ARE marimba pieces where rolls have note values. Usually with those they use a "Z" for the unmetered roll (like a buzz roll on a drum) and the slash roll notation for note values, where one slash is 2x the note, two is 4x, three is 8x, etc. (again, just like rudimental drum notation). Pretty rare though, and there's usually a composer's note in the score to clarify these.
I've also seen marimba rolls using dots underneath to show the subdivision, so for example a quarter note roll with 4 dots = 16th notes, one with 8 dots = 32nd notes, etc.
But unless specified, they are unmetered and played to best fit the song.