Depends on a few factors, but here's my 2 cents:
Grab a bunch of books and your favourite grain magnifier. Go to your enlarger and put the grain mag on the baseboard, now, keep adding books until you can comfortably look down into it (be aware time of day has an impact, so shoot for the time of day you are normally in the darkroom) when you get to that point, you should find youself standing pretty straight up with little strain on the back and neck. Now (as with everything in photography) you must begin the compromise... if you moved the enlarger this high, do you still have clearance above? Can you comfortably work the controls? Is getting paper in/out of the easel still relatively easy? If the answer was no, start compromising until they are yes

You will most likely find a sturdy little 1' stepstool will now become handy to have under the counter for those extra-large prints.
Mine are about 39" and the framing counter is actually about 41" - I am about 5'8" and I can stand there and print without getting 'the darkroom stoop'
-Brad