Use another entity called multi_manager with it's "multithreaded" flag ticked and which basically calls itself, two env_renders, one which "shows" the entity, and one that "hides" it. E.g. if you have want to toggle an env_sprite, set the env_sprite's rendermode to "Glow" and the amount to e.g. "255", then for the env_render that shows it copy those values and for the one that hides it change the amount to "0".
To create a multi_manager, just place it as a normal entity, edit it's properties, give it a name and set SmartEdit to off, then just add the name of the env_render that hides the entity as a key and for value give it 0.1(I read that using 0 can cause bugs), do the same for the env_render that shows it but this time set a delay of 1 (or how fast you want it to flicker). Then add another key with the name of the multi_manager and the delay should match with the last key you added. Then switch to the flags tab and tick the "multithreaded" one.
However I'm not sure if this will work with every entity and I read that using "multithreaded" multi_managers for more complex stuff isn't really efficient, as it basically just creates a copy of the multi_manager until you hit the engine's limits. There's a way to prevent this though: https://twhl.info/index.php/wiki/page/Tutorial%3A_Multi_manager_Looping_Made_Easy
An easier way to "loop" this would be using an env_beam, but it's "random": https://twhl.info/index.php/wiki/page/Tutorial%3A_Random_entity_triggering