Thnaks to all for the answers.
Sorry for not being clear with my question.
Im trying to build a controller for a synth, not a programmer student, so my knowledge is sketchy.
Ruilviana got it for me. I meant how much data can I get ready to process in the loop function.
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).
Thank you all for your time.