the で here says when something is happening, and the “let’s think about” bit is left implied (it’s an incomplete sentence).
from imabi:
- で shows an extent which may create juncture. Juncture deals with a point in time or place. However, it is not definite in nature as the particle に. This does not mean phrases like 一秒で are impossible. The purpose of に is to show exact time. Think of the difference as “The ice fully melted in 3 hours, 4 minutes, and 32 seconds” vs “The ice fully melted in three hours”. Juncture could also be used to show at what point something happens. The vagueness of this comment is on purpose. For instance, you’d use で to show at what temperature something boils or melts.
but regardless, it’s a set expression anyway:
- we’ll cross that bridge when we get there; we’ll think about it when the time comes


Not sure about that though.
