I generally go fast enough that I'll arrive without being too pooped to climb 3 flights of stairs to the office.
At my last workplace, my journey time was around 35 minutes. If I went balls-out, I could get there in 30 but I'd take 15 minutes to cool down and then need 10 minutes for a shower. I go faster
I can't cycle much slower than I do, though, I find it harder to work at a slower rhythm than my natural pace.
That said, I left here on Friday after my wife reminded me I needed to be home in 35 minutes to look after the kids. The previous best was 36 minutes and I did it in 32, although I thought I'd blown it half way.
So, generally, I go at a fair pace (for me) which keeps me breathing hard and my legs know they are working, but not flat out.