Approximately how long after starting with a Major airline does it take to gain the seniority to fly a widebody aircraft? FO vs CA?

Very roughly, F/O a couple of years, Captain a couple of decades. Will vary quite a bit by airline. Wide body Captain usually needs to be in the top 10-15% of the seniority list.

And remember too that you'll probably be on reserve for a longggg time if you take a WB slot as soon as your seniority allows. In base this might be a plus actually though.

Day 1 at UPS.

For legacy pax airlines, it was traditionally about ten years for FO, several decades for CA.

Today you can get FO in a few years, maybe even as a new-hire if lucky in the class drops.

CA is hard to say, the current retirements will move things along a bit but I think you're still looking at 15-20 years at a minimum, even if you hit it just right.

I would only worry about wide-body vs. narrow-body if you can get to a legacy by your mid-thirties.

Using current junior CA awards at AA and future retirements -

767 CA - 2030
330/777/787 CA - 2032

Hopefully someone double checks that because it seems too fast.

2022/2023 for junior w/b FO.