You might sometimes build an app where (through your operating system) you connect directly with an input device and/or output device and then do all the audio processing yourself. In this case, you'd more or less control the whole bus and all the code processing samples on it and have a fairly true sense of your deadline. (The OS and drivers would still be introducing some overhead for mixing or resampling, etc, but that's generally of small concern and hard to avoid)

Often, though, you're either going to be building a bus and applying your own effects and some others (from your OS, from team members, from third party plugins/libraries, etc) or you're going to be writing some kind of effect/generator that gets inserted into somebody else's bus in something like a DAW or game. In all these cases, you need to assume that all processing code that isn't yours needs all the time that you can leave for it and just make your own code as efficient as is reasonable.