How to estimate the altitude of an aircraft at a given distance from a runway?
Straight in, on glidepath, about 6000' above the runway. Straight-in &amp; fast, below glidepath at 3000 to 4000 feet above the runway.

That's based on about 3 miles per thousand feet is a standard glidepath, and it takes in the ballpark of 5 miles at idle power &amp; level flight to slow from 250 knots to a configuration for starting down the glideslope. 

Those are ballpark numbers; a heavy aircraft slows less rapidly than a lightweight one does, headwinds can shorten those distances, while a tailwind at altitude can extend them. And there are techniques to slow a little faster than that, and you can leave the power in &amp; slow down over a (much) longer distance if desired.

But for ballpark values, those are good approximations.