How to control sample duration during data acquisition

I managed to collect 16 channels of analog signals (write to measurement file VI) manually using a boolean start/stop control.  I would like to allow the user to set a sample duration on the front panel (e.g., 20 seconds) and initiate the sampling by clicking a start button.  The user can also terminate the data dump before the end of the set duration by clicking a stop button.  I looked at several countdown timer examples but couldn't figure out how to incorporate it to my VI.  Any suggestions and examples will be much appreciated.