My understanding is it can sometimes be less. 2-3 weeks...so maybe January gets rocking and rolling. If it happens at all, of course.
A few thoughts here...per Simon Lee's research, a -NAO is 7 times more likely to occur when the lower stratosphere (100mb) is in a weakened state.
There are multiple ways to get the Strat PV weak at 100mb. One way is to have an SSW. SSWs are designated / occur in the upper stratosphere at 10mb. So if the SSW occurs at 10mb, then downwells into the lower strat at 100mb, that would move the 100mb layer into a weakened state. The downwelling into the lower strat may take 2-3 weeks. In some cases, the downwelling associated with the SSW isn't effective, and the weakening in the lower strat is minimal.
However, if the lower strat is already in a weakend state when the SSW occurs, the downwelling is essentially instantaneous, and it simply magnifies the lower strat weakness over the next several weeks. This is the ideal evolution for those who like winter and is what occurred in the 2020-2021 winter when Texas/OK were hit hard with cold, snow, and ice.
But even without an SSW occurring, the lower stratosphere can be in a weakened state. Take the big -NAO in Dec 2010 / Jan 2011. Here we can see on the chart below the lower strat at 100mb in a weakened state in the Dec to mid-Jan timeframe (i.e. red 100mb zonal wind line on the chart), and no SSW occurred that winter. Cases of the lower strat being in a weakened state in absence of an SSW typically occur when the strat PV is getting attacked with rounds of heat flux from waves upwelling from the troposphere, but not in such a strong manner that it leads to an actual SSW.
Bottom line, the most fertile environment for high latitude blocking to occur is when the lower stratosphere is in a weakened state. An SSW isn't a must in order to have the lower strat in a weakened state, but it's one way to get there if the downwelling environment is suitable following the SSW.