I went back when LO was 20 weeks, I am a teacher so am in school by 8am, I try and leave around 3.45 (so work though that time without stopping, even eat my lunch while marking books), then I work for about an hour or so in the evening planning or marking (so that's just over 8 hours a day) Plus I then work for about 4-5 hours on a weekend early morning and in the evening. So about 45-50 hours a week.
Last week we had OFSTED so I was 10 hours a day in school, plus more at home in the evening, then I had about 5 hours at the weekend (so just over 60 hours in total)
Report writing time, parents evenings and summer concerts will all mean more hours.
So a lot!!
DS is home with DH though, so that makes it a little easier to bear. It's hard though.