This really depends on the bit you are using and what the flute length is.  If you are using a bit with a 0.5" flute length then your maximum thickness would be 0.5".  Keep in mind that when you go thicker than 0.125" thick aluminum, you have to be very careful to take care of chip evacuation. When your end mill starts to make a deep channel that is the width of the bit, then chips will fall in the channel and when the bit rotates, it can recut the chips causing excessive heat, chatter, welding of the chips to the tip of the bit or breaking the bit.  A way around this for deeper cuts is to create a pocket around the perimeter that is wider than the bit so if chips fall in the channel they are not "forced" into the wall by the cutter.  You should also use compressed air to continually blow the chips away from the bit and the channel.
We have successfully cut through 1" thick aluminum blocks without a problem. Keep in mind that this is a CNC router and not a CNC mill, so it is not designed to be as aggressive with your feeds, speeds and pass depths.  Take care to not push the machine, spindle or end mill past its limits.