Did you try searching the forum? This has been discussed a few times.
Here's an excellent thread with experimental results (and explanations) of how quickly an Arduino can toggle a pin. http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1230286016
Of course the answer depends on how
you toggle it - using arduino-native digitalWrite() commands is much lower than using assembly code.
If fact you could
get it to toggle millions of times per second - 4 million times if you use a 16MHz clock and assembly code. Using digitalWrite() would get you one hundred thousand or so per second.
Even if an LED could toggle this fast, your eye wouldn't be able to see it :-)