When I stopped playing BG1 a few months back I had just entered the city itself.

I'm thinking of restarting, but I'm wondering how far the game I actually am.
50%? 75%? 90%?

I rather want to start BG2, but I kind of want to keep my characters and see the end. Not sure if i want to dedicate another 40 hrs to it though.

I don't know how many hours it would be, but there's a ton of stuff to do in the city. If you follow just the main quest line you can get through relatively quickly, but if you want to explore all it has to offer, then you're looking at quite a few hours.

It entirely depends on how much exploration you did before getting into the city. If you just did the main quest so far, you've done maybe 15% of the game, while if you've explored all the zones accessible before BG thouroughly, without doing stuff from TotSC, you've done about 50%.

Either way, you can finish in about 5 hours if you want to stick to the main quest only, but doing the optional quests in BG and TotSC content adds a good 20 hours to that.

It depends on exploration and gaming style. Say that you explore moderately (i.e. about half of the wilderness and half of the quests done), and haven't touched the ToSC content (which I do not blame you as you probably are not prepared for them, especially Durlag's Tower), then you probably saw around 30~40% of the total content.

For many people, including me, it takes a lot of time to finish some of the Tosc contents. It took me probably 10 hours to crack through Durlag's Tower as my party was wiped out many times and there were certain locks and tricks I could not figure out so finally I had to resort to walk-through, and I never beat Arc Artex (or whatever its name is). Some battles in the city are quite difficult if you are unprepared. With so much said, I believe that if you are skillful or are willing to horde large amounts of potions and wands you should be able to beat each difficult combats within 3 reloads.