Both can be within a rhythmic value or done freely. Can honestly say I don't know of any music with trills/tremolo that isn't done as semi-quavers or a faster value. At the same time, they can both work easily not being set to a value per beat.

I think it's all about what the music needs for whether a trill/tremolo is or isn't in time.
I have a tendency to "peck" and thus cannot get a good tremelo going. anyone have a quick fix or suggestions. Been playing bluegrass and some swing for many years