I started back to work full-time (37.5 hrs/week) when my daughter was 4 months old. I worked every day from 7 to 2:30. My husband took care of her in the early morning, and then dropped her off at his parents around 9:30 or 10:00; I was able to pick her up around 3. She only needed 2 bottles a day (which meant only 2 pumping sessions), and I had long afternoons to spend with her; she was only without a parent for about 5 hours a day.
Starting in January, I started working a 4-day week; 9.5 hours Mon-Thurs, with every Friday off. The first four days of the week are really long and exhausting, and I hate only spending a few hours at the end of the day with my daughter. However, having a long weekend every week is wonderful. I don't think I would have been comfortable with this schedule when Mallory was a tiny baby and was nursing more. (We still nurse, but not as frequently and she's okay with waiting for me to get home on my workdays.)
So, what worked for me was 5 shorter days while she was a "baby" and 4 long days now that she's a "toddler."