Complicated question. It partially depends upon where the power is coming from, USB or external power.
If USB, the computer limits the current to 500mA, though some of that (~30mA) goes to the Arduino itself. So you can probably count on ~450mA or so for your own work.
If external power, current comes through a linear regulator that gets hotter with increasing current and difference input-output voltage. If it gets too hot, it shuts down. I generally suggest keeping linear regulator power dissipation below 1W, and if your output current is 500mA then input-output voltage difference should be no more than 2V, so something very close to 7V-8V is required...any higher and the regulator may overheat. There is some heatsinking on the PCB around this regulator so you may be able to push it above 1W, but we'll see....
If you need less current then you can tolerate higher input voltage (external voltage). For example, if you will draw 200mA then at 1W, the regulator power dissipation is (Vin-5V)*200mA=1W and Vin is as much as 10V.
The Arduino Drum Machine
: 14-track MIDI drum machine sequencer / groove-box