The duration that you specify in your dynamic simulation defines the amount of time that the explicit code integrates.
How long it takes the solver to run the simulation depends on your study set up. The explicit dynamics integration method uses very small time steps to integrate through time, and the run-time is directly proportional to the number of steps you specify. It is common for the time step to be on the order of a microsecond, and it is not unusual for it to be a nanosecond! Durations on the order of a second, which would take millions of times steps, are not tenable in an explicit dynamic simulation.
Prior to an analysis, the software calculates how long the analysis is likely to take. If the estimated run time is too long, the solver will tell you so you can take action.
There are several actions that you can take that may significantly reduce the estimate: