When I'm waiting for a pc game, I like to switch things up and play some console sports titles. Career mode can suck up a lot of time. If you're wedded to the pc, I could think of a few games that would eat up a month:
Europa Barbarorum 2
XCom Long War (even the short version can run you 100+ hours)
Grim Dawn
Distant Worlds
Panzer Corps Grand Campaign '39-45 (probably take you to August to finish).
Order of Battle Pacific
As others have mentioned, WitE/W
Feeling old school: John Tiller's Campaign Series or maybe TOAW3 (though I never played the latter). I really loved JTCS despite it's various flaws. I especially enjoyed the campaigns: Big Red One, Fallschirmjager, Screaming Eagles (captain Winters is one of your actual commanders!!), Operation Rumyantsev, Battle of the Bulge (as Germans), Kokoda Trail... I could go on and on. You'd be playing all those for like 6 months. Even the East Front tutorial missions are great fun: Operation Typhoon has stalled out and your elite ski infantry have just arrived from Siberia. It's time for a counter-offensive!