Here's what 1000 feet on the same track looks like:


2000 Feet for the same-direction traffic above, 1000 feet for the opposite direction split-down-the-middle aircraft. It "looks" a lot closer than it really is.

Edit: And it's 1000 feet in "Flight Level" altimeter (29.92 inches of mercury at ISA). It could be more, it could be less. It depends on the local isobar and temperature.