Musically, how long is 16 bars?

Like both in a sense of time and of counts (8 counts, etc).

Time depends on tempo. Beat wise, 4/4 time would be 64 beats

In 4/4/ time it's 64 beats. In 3/4 time it's 48 beats. What's your point?

It depends on the time signature and the tempo of the piece. So, you'd need to provide that information.

It is usually about 30 seconds depending on the tempo

It all depends. In 4/4 time, it's 64 beats. That's common time, and mostly used in alot of music.

:) Hope I helped!