https://blog.torproject.org/announcing-vanguards-add-onion-s...
Tor is a tool like any other. It has certain strengths and certain weaknesses. When you're evaluating any security product you always have to determine if the security properties the tool provides match up with the security properties you need. Tor is no different.
We struggled to find a commonly accepted term for vulnerabilities at this stage of their life cycle, but we finally settled on n-day vulnerability. This term have been relatively well accepted by the vulnerability research community.
The exact length of this period is completely dependent on the velocity of the community to adopt a mitigation such as a patch. Heartbleed and Shellshock had been massively mitigated in a matter of days or weeks, but EternalBlue based-attacks still caught a lot of production systems off-guard more than a year after its disclosure.