From Ren'Py

Auto-read Setting

To enable and set the delay of the auto-read mode at the first run of your game (instead of going in the preferences menu), add to your script :

init:
     if not persistent.set_afm_time:
         $ persistent.set_afm_time = True
         $ _preferences.afm_time = 10

This will change the setting to 10 the first time the player runs the game. It will then store the fact that it has done so into game/saves/persistent, so if the user changes the setting, it won't be reset.

Maximum setting is 41 for the default Menu Preference User Interface, but it can be set higher in your script file. Minimum setting is 1 and will make your game show all lines of text with no interruption.