There is a lot of confusion about this topic but Ohm's Law can help us here. The amount of current demand your system will have on your charging system depends upon the amp and your listening habits.
First lets look at what your vehicle needs to run. Depending on which and how many accessories you are using, you vehicle will need 40 - 60 amps to operate.
Your amp is rated for 1500 Watts RMS @ 14.4V. Current (amps) = Power (Watts) / Voltage. To figure out how much power an amp rated @ 14.4V would do at realistic in-car voltage, multiply power by .7. (1500*.7= 1050). So lets's plug this into the formula: 1050 Watts/12.5 Volts = 84 Amps.
This is not the whole story hower. The number above assumes the amp is 100% efficient, but it's not. A design like this amp has great efficiency but when you consider the greater levels of clipping the amp will be exposed to it's efficency is about 50%. This means it will need 2100 Watts to generate 1050 (yikes). So lets see: 2100 Watts/12.5V= 168 amps plus the 40-60 amps you car needs, you need a 200 - 320 amp alternator just to feed this one amp....or do you?
You are the other factor. Your listening tastes dictate how much of that availabe power you are going to use with music. Most people will use 20% of the amplifier's power on average with music:
1050*.2=210 Watts (average)
If you are like most people, you probably will not need to change the alternator if it's healthy.
Abusive poeple will use 50% of the amplifer's power on average with music:
1050*.5=525 Watts (average)
Probably should upgrade.
The major difference between these two types of listeners is the amount of clipping they will drive their systems to. Most people will drive thier systems into clipping, some more than others. Clipping dramatically increases average power.
There are other factors that influence average power like Crest Factor of recordings but that is a different discussion.