I have had the same experience as you. There was no significant 'teach-in period' when I picked up a replacement spare key for my W209 CLK in 2006. I arrived in the car and the new key worked immediately.
I'm not really aware of all the details here but my understanding is that different observed teach-in periods are from different types of keys.
Fresh MB cars have 8 key tracks and and each track has 3 segments. Only one key can be active on each track. The car comes with two keys, one active on the first segment of the first track, the other key active on the first segment of the second track.
If a key is lost, the dealer may order a spare key or an extra key. The spare key is coded to the second segment on the lost key track (or the third segment if this is already a second replacement key for that track).
The extra key goes for the first segment of the third track (or any higher track if extra keys have been ordered previously).
The teach in times mentioned previously are needed when new hash code tables are calculated. If I remember right, the extra key works immediately because the EIS already has the necessary hash code tables for this key and the key is prepared accordingly. But the spare key needs a new set of hash codes calculated and this takes a lot of time.
Of course customers are asking for a spare key but the dealer should order an extra key, the customer should not need to know anything about the difference between the two or even the existence of such a difference.