PaulS:
How exactly should I go about with the parameters of this function?
That depends entirely on what you want to read until. What marker will cause you to stop reading?
Well the parameters are Serial.readBytesUntil(character, buffer, length). They seem a bit unclear to me. It seems like it is asking to stop reading at a certain character and record a certain length of these characters to a buffer. I want to stop after the last character typed and record the whole thing to pulseWidthInput[], which will then be turned into an integer with atoi().