This has been talked about a while.
The average season length has been a shade under 101 days since the inception of the idea. If we exclude the last few seasons when things were complicated by the pandemic and working-from-home etc, the average drops to 96 days. Not far off your 3 month suggestion.
See this post 
, too, with a list of a summary of the season themes. Average season length is 107 days for all themed seasons, but drops to 98 when I deselect the last few pandemic-impacted seasons.
Once they’ve finished developing seasonal themes then I imagine it’ll settle down to 3-monthly. I expect by then D4 will be upon us. Or the world will have exploded.
PS Statisticians please don’t @ me about my lazy statistics usage. 
PPS a lazy graph for y’all, too:
