Time-wise 40 min is probably around average for Toronto (door to door). Lots of people have shorter commutes who just walk, bike or drive quickly. However lots of people also have longer commutes, commuting North York or Scarborough to downtown by TTC, or taking GO would probably give 1-1.5 hour commutes. If you're doing midtown (Yonge-Eg), Bloor West or Danforth to downtown by TTC it should be around 30-50 min. Places by streetcar to downtown probably take a around the same too if you take walking and waiting into account. Talking to people a 40-45 min commute is very common within Toronto.
However so far for me it seems OK because half of that 40 minutes is walking in walkable areas, which I find enjoyable. I prefer that to spending the full 40 min on the subway, or driving on the highway, although winter might be a different story. I could try to shorten the last 10-15 min walk by taking bike share from the subway, but it doesn't really seem worth it so far over walking.
Most financial jobs (or jobs where you need to wear a suit, which to me seems insane for Toronto's hot & humid summers), seem to be around King-Bay down to Union area, which seems very convenient in terms of GO, TTC being directly connected through PATH. However I'd say a big cluster of jobs is west of there from University and Spadina in the entertainment district. Commuting to there from the subway you'd always have a 8-15 min walk tacked on to the end, and the streetcars don't really speed it up much.
Often in other areas like Bloor West the office is still 10-15 min walk away from the closest subway station.
In terms of why a commute would change, the most common reason would be job changes. Many young professionals switch jobs every few years (or even more often). Some people do major switches like Markham to downtown, but even switches within Toronto like Yonge-Dundas to Yonge-Eglinton or Dundas-West/Bloor to King-Spadina can cause commute time changes. Obviously, you can move along with the job, but sometimes people do or don't for various reasons.