I think its how long is a piece of string. It depends on what job you do.
I work in a professional, client facing role and I am one of a tiny minority that worked a 3 day week (which I've done for 7 years, just gone up to a 4 day week now youngest is at school). The vast majority of mothers in my firm work a 4 day week (or full time). There is little flexibility for half days / working from home etc. Normal full days (so 8.30 - 5 / 9-5.30 etc). Children were in nursery from about 8.15 to 5.30 (if I did drop off, H would do collection and vice versa).
At school, 3 day weekers are still a minority I think. There are definitely more parents who have given up work all together (or started their own business which they can work around school) and definitely more who do a 4 or 5 day week.
At my nursery, there were only 2 options when I started DD there a 3 day place or a 5 day place. They now offer a 4 day place too. No half day slots etc.
For me, on the good days, 3 days was great. DD was with me 4 days, nursery 3 days, could still argue she was with me more often than she wasn't. On the bad days, I was doing half a job at work (and completely overlooked) and half a job at home. Its kept my hand in though, DD loved nursery and the balance was just about right. 4 days is tough (with 3 children at school) and even after a relatively short time, I'm considering my options.