To be clear, there’s no direct evidence for a timeframe.
However, there is tons of evidence that shows that Gaijin is actively working towards adding submarines. A lot has been posted further up in this thread. Antisubmarine weapons for ships, depth charges for aircraft, submarine voice lines, and submarine spawns for naval maps are already in the files. So it’s clear that they will be implemented at some point, we just don’t know when.
There was even French submarine voice in the files slightly before the French navy was confirmed by Gaijin, so i have nice hope for subs being a real thing ingame “soon”
And since the last event was German sub, and than the older event there was no French sub ( sad French noises), this is good hope for subs in WT because it’s not just left over from older events, things are really worked on.