How many horsepower does an alternator take from an engine at different speeds?
The efficiency of the alternator is terrible under most circumstances. 

The Alternator itself is only usually about 50-55% efficient. For example a 100A alternator will provide an output of about 1.5kW at 15V, but will dissipate about 750W internally. That's why they have such a big fan on the front of the alternator.

If you then consider the mechanical power required to spin and cool the alternator your overall efficiency drops to about 20-25% including the engine losses.

If you like to deal in HP, then 1.5kW is about 2HP. At an overall efficiency of 20% this means you will need about 10HP from an engine to provide this.

You could read this report from Remy which lays out some of the losses. You will notice that the losses are dominated by the engine and mechanical drive system. From the report this graph may address the alternator losses alone:



The amount of power at idle, normal and maximum is alternator dependent. Most alternators need to spin at about 6000 rpm or more to provide maximum output, and in most vehicles you will see max alternator rpm around 8-10000 rpm. 

The efficiency (and power output) of an alternator can be improved markedly by moving to synchronous rectification and raising the output voltage using load dump configurations. For example a 48V 100A output may improve to almost 75% or better.