dear james
well there is no "hard" limit... the limits are imposed by the responsiveness your application needs
for example... even if you can collect 1000 sensor reading at once, you still have to send them to the computer.
so the speed of the serial connection and the protocol you use becomes very important...
consider also that max sometimes doesn't like to receive a lot of data from the serial port.
Briefly the answer is depending on the way you design your circuit and your patch the sky is the limit 
massimo