I’m not sure what the parent was referring to, but the simplest and most common queue model uses a poisson process[0] for both queue entries and exits, which means inter arrival and processing times are both exponentially distributed. This is called an M/M/1 queue,[1] and waiting times can be analyzed with Little’s law, though that breaks down at the inflection point where service times are identical to waiting times.

[0]: https://en.m.wikipedia.org/wiki/Poisson_point_process [1]: https://en.m.wikipedia.org/wiki/M/M/1_queue