The time at which something happens.
At what time? At which time? Upon which occasion or circumstance? Used to introduce direct or indirect questions about time.
At (or as soon as) that time that; at the (or any and every) time that; if.
What time; which time.
That's enough, a command to stop adding something, especially an ingredient of food or drink -- referring to say when.
The word "when" comes from Old English hwonne, which originally meant "at what time." It traces its roots back to the Proto-Indo-European interrogative base kʷís.