We lock threads that are more than 90 days old, essentially 3 months. By the time 3 months have past with no one commenting in a thread the topic is normally outdated and is not suitable for bumping back to the top of the boards. In the case that a thread is locked when it shouldn't have been you can PM any mod and they will be able to restore it to the top of the boards so everyone can post again.
We have this policy as from experience old threads that people find via the search end up coming back when they are outdated and just confuse people.
Windows 7 Pro, Intel i7 3.8 GHz (Passmark: 9021), 16GB DDR3, NVIDIA GTX 780 4GB Superclocked (Passmark: 8056)