For a stimulus to be displayed for certain duration, write the time length in miliseconds under the time column. If you want the stimulus to display until a key is pressed, just specify the valid response under the time column instead.

For instance, for a flower to be displayed for 500 ms and a sun to be displayed until the spacebar is pressed:

stim | loc | time
flow |0,0,1| 500
sun |0,0,1| rt:57