Depends on the game itself.
I played Diablo 2 offline for about 3 years and a half, originally (around 2001 or so when I started it). Then went B.Net / Online for another 3 years. Then something like Natural Selection (Half-Life mod) was about 90% multiplayer, did some occasional offline sessions with Bots. If I go back to maybe early PC gaming in general it was maybe half and half. I played a bunch of Battlefield 1942, Desert Combat (the original mod) and then Battlefield 2; all of which were also about 90% online (and again, I think I did play some offline with bots). If I think of something like Command & Conquer 2 (Tiberian Sun + expansion) it was half and half, same with Red Alert 2.
Something like Unreal Tournament 2004 was about 50/50 for me, I played a bunch of Mutators and Invasion mods both offline with bots and online on specific servers. I'm skipping a lot, but I'll jump right to something like Mass Effect 3, which I played the campaign of only one single time (was more than enough, actually it was too much) but I ended up playing about 2000+ hours of its MP mode. Some years before ME3 was all the rage with Team Fortress 2, that one I played about 100% online, similarly so with the first Left 4 Dead (although I did play some offline sessions with bots here and there).
Right now I play Heroes of the Storm regularly, only online (well I mean sure, we need to be online to play it; although we can play it solo with and against bots, but I do play it with people Versus bots). I also started the Iceborne expansion for Monster Hunter World back in last December, and even though initially I played it solo (offline sessions) for the main campaign quests I'm now splitting my time with it equally between offline and online sessions, and I sometimes join SOS flares (joining someone else's match to help them) or ask for help when I need it.
Even if I go back to my first ventures with 'Online Gaming' in late 2000 on my Dreamcast, I played the games that did have online about 50/50 with offline too (Quake 3 Arena and Phantasy Star Online for example were two of those). But if I go with say... Dragon Age Inquisition (which had a multiplayer mode) then I skipped MP entirely and exclusively just played the campaign offline.
So ultimately yeah, depends on the game, and overall I'd say it's about 50/50. And I skip a game's MP mode(s) if I just happen not to care for it (it does happen here and there).