It's pretty clear from the description that the LTB cycle is variable, dependent on multiple factors and completely at the discretion of SonicWall, so I don't see how anyone here can give you a definitive answer.
Last Time Buy (LTB)
Last Time Buy (LTB) is advanced notification to
SonicWALL customers and partners that SonicWALL intends to start the
end of life process. The duration of this phase is variable and
depends on numerous factors including material availability, SonicWALL
and channel inventory and end-user demand. Last Time Buy is
informational only. Products in this phase are active.