The duration of each chord is one bar a.k.a. one measure. This usually means:

4/4 time signature 3/4 time signature
4 strokes (eventually 2 or 8) 3 strokes (eventually 6)

For instance, in a 4/4 time signature, the chords Bb Dm F F mean:

  • 4 Bb strokes
  • Followed by 4 Dm strokes
  • Followed by 8 F strokes