[Pescador]

Confirmed: The swing timer does not reset when you cast - it continues to count down while you cast. On a paladin, you can time it so you can cast flash of light in between soulfire swings (haven't tried it with max haste yet). If your timing is off, you'll miss a potential swing.

So the swing timer starts once you turn on autoattack, and does not stop or reset for stuns, casts, or out of range.