You can use a Relay module to gate when data is allowed to the Write Data module. A trigger module, such as the Combi Trigger, can be used to control the Relay. Set the Stop condition to Directly and the Duration to how long you wish to keep the Relay closed. To see it in action, also connect a Chart Recorder to the Combi output. If you set the Write Data module to use the Multi File option, you could also use an Action (requires DASYLab FULL or PRO) module and the Close and Reopen action to instruct the Write Data to begin a new file. In this case, you would set the Action Event to the falling edge so it creates the next file on the falling edge of the Combi trigger after the data was written to the file.
