How long do you estimate it takes to finish this game for a first playthrough, with recommended mods?

I'd say 20-40 hrs for the main game, much depending on your playstyle.
I'd spend enormous amounts of time optimizing character inventories and spell memorization and hauling semi-worthless loot back to nearest shop, Enough to pale the amount spent actually fighting, exploring and advancing the plot.

The new content adds almost as much more, but after several playthroughs I still haven't done them all (and I doubt I ever will).

Thanks for the reply.

It's not a long game at all, 20 hours at most. The extra content (which I do not recommend the first time through) adds on another half dozen or so.

Why wouldn't you recommend it for a first playthrough?