As I told you in chat,
While the glass brush is selected, hit Ctrl + T. this turns the brush into an func entity, change it from a func_detail to a func_breakable, name it, and click the Outputs tab. Click Add.. OnBreak glassTemplate ForceSpawn (after a delay of) 30.
Now create a point entity as a point_template. change its name to glassTemplate and in one of the template keyvalues place the name of your func_breakable.
So the whole thing goes: glass onbreak template forcespawn, which spawns in another glass 30 seconds later.
Edit for the sake of information:
you're going to need another point entity called logic_auto outputs onMapSpawn template forcespawn since point_templates aren't autospawned