how long to allow for emigration at Madrid airport?

From the time you land and exit the plane until you are at the baggage claim, one hour max...more likely one half hour....the most time will be in walking from the plane to the processing station....and depending on the gate, the stairs, etc., it can be a hike....When I arrive at sometime around 830 - 9 I can usually be to the hotel (hostal) by 11 by taking the aeropuerto van service. and this is about a 1/2 hour ride.

We landed in November 2012 at Barajas Terminal 4. It was a civic/religious holiday and the terminal was basically empty, which was eerie. We walked, and walked, and walked some more, finally reaching some elevators, which took us to the passport check. It seemed like we were there a really long time, and had there been other people standing in line (we both stopped at the restrooms after getting off the plane, and everyone else must have run ahead of us, as the place was, again, virtually empty as we did the looooong walk), it could've taken even longer, but we breezed past the passport desk in a few seconds, as we were the only passengers there at the time. Weirdest airport arrival ever. Depending on your arrival terminal and your walking speed, you may need to allow extra time!

Immigration/passport control takes very little time. The last time I flew in there, I had checked a bag. Even with the waiting time for that, I was out in roughly 30-40 minutes. Then, as HJ said, it's about 30 minutes into town.

Actually the tailwind typically favors your arrival and I have never been late, often arriving early, on flights into Spain.
As a result we have caught connecting flights scheduled only 90 minutes after initial arrival.