So I'm assuming you mean that you want the Simulink model to run for 15 seconds of real time. Otherwise if you just want it to run for 15 seconds of simulated elapsed time you could just set you stop time to 15 seconds.
To make it run for 15 seconds of real time you can use the Simulation Pacing Option. To get to this setting click on the drop down below the Run button. There you can check the box for "Enable pacing to slow down simulation, and set the slider for Simulation timer per wall clock second to 1. With those settings, if you set the stop time of the simulation to 15 seconds, the real elapsed time will also be 15 seconds.
