I agree fully with Jim. There is no simple answer to your question, and as he says, a 2 flute cutter would be better for the job.
A two flute will cut more material per flute than a 4 flute, however since you are cutting aluminum, the more material cut will not matter too much to the cutter. I have rarely broken a cutter on aluminum (once if I recall correctly…). As long as you keep the spindle speed and the feed rate right, as well as use proper cooling, the cutter should be fine on even heavy cuts. Also since there are only 2 flutes, there is more of a relief groove on the cutter, so the chips fly out easier, preventing blockage.
Remember, different materials have totally different cut techniques. For example, I would always cut steel with a 4 flute cutter.
As for the original question, it is hard to say with the info you have given us. If you have a mill that is similar to a Bridgeport Series 1 mill, and in decent condition, you are in good shape to do heavy cuts. If you are running a half inch cutter, 2 flute, HSS, you can probably get away with cutting .250" deep with a 3in/min feedrate. That is, if you keep it constantly cooled. If CNC, you can mostly only judge by the sound of the cutting and the heat produced. If non-CNC, you can usually just go by the seat of your pants and judge the feedrate by the feel you get through the screws.
The main issue with cutting deep is that the part you are cutting has more of a tendancy to be sucked up into the cutter. If you have a part viced down very well, you can cut normally. But if you have a wierd part that isn’t held on too well, be very careful. Take cuts as small as .050" per pass, or lower the feedrate, depending on which direction the part is more prone to slip.
The art of feedrates and cutting depths is hard to perfect, it takes many years to learn, and can’t be easily explained. The more experiance you have on a mill, the better. Hope that helps.