There is no "Void loop"!!!!! There's a function called "loop". It has return-type void.
The question is meaningless as the loop function could take any amount of time to run, from < 1µs to whatever you program it to do - so I think what you actually need to ask is "what's the maximum execution time I should make loop() take so I don't lose incoming serial data?"
And the answer to that is given by the serial buffer size times 10 divided by the baud-rate. 64 bytes and 115200 give 5.5ms. The 10 is the number of bits in a serial character (8 + start + stop).