Dear ef_c.
You should be able to accomplish this by using the skip features in your response file and the MinMax field of the Styles file.
Using the skipping feature is very straightforward. Just define whatever keypresses using the rt: feature in the time column of your input file. Take a look at the Input File > Skipping and DirectRT Tutorial > Ordering Trials > Skipping sections of the DirectRT manual for more information and an example file.
To limit the amount of time for the trial, you could use automatic continuation feature of MinMax field in the Style File. Enter 0-xxxx! in the MinMax field, replacing xxxx with however many milliseconds you wish to display the stimulus for that trial. If you need to display stimuli on different trials for different amounts of time, you can do so in a different row of the style file. Take a look at the Style File > MinMax-Setting RT limits section of the DirectRT manual for more information.
Please let me know if I can help with any other questions you have as you put you study together.
Cheers!
Jason
Jason Reed
Empirisoft Software Support Specialist