Conducting a continuous fire scenario

In a continuous fire scenario assuming that the duration is 60s and the rate of fire is 2(rounds per minute); it should fire the first round at 0s, the second at 30s, and another round at 60s. Likewise in a scenario which spans for 90s with the rate of fire as 2 there will be rounds fired at 0s, 30s, 60s and 90s.