Where does it end? If we accept that the prime minister has the power to determine when the parliament, what is to stop a prime minister from proroguing parliament for weeks, months, or even years at a time?
The prime minister does not have unlimited legal authority to prorogue parliament indefinitely. They are constrained by legal precedents and the constitution. There is no legal precedent for the prorogation of parliament for 33 days. Nor is Johnson permitted, within the constitution, to prorogue parliament for the purpose of stymying debate and bypassing the democratic process.