If you roll a one, your effective damage is 0 5/6 of the time.
If you roll a 2,3,4,5, or 6, then your average damage per roll is 4.
If n is the amount of turns, and k is the expected damage, you come with:
Maximizing this function gives the optimal number of dice rolls to to be 5.48, or about 5.
Multiplicative effects do not change this as long as such effects is constant throughout all rolls.