Following J-SP's excellent advice, the hours are unpredictable and it does vary considerably depending on the department you're in.
What you will hear often is that the hours come in peaks and troughs. The severity of the peaks and troughs depends on the nature of work of the department and the volume of work coming in and a little bit of luck. A friend of mine in Debt Finance gets so immensly busy when he is busy that he works 7 days a week and anywhere between 12-18 hours/day for 2 weeks straight. However, when it gets quiet, he is literally reading the Guardian and checking posts on Facebook and disappearing at 5.30 on the dot because he had had nothing to do all day long.
Compare that to my seat in a contentious team and the latest I ever worked was one lone night when I was in until just after 11. Ordinarily, I was out the door between 6.30-8 on any given day, however, I was usually busy all day long and there were very few days when I found myself twiddling my thumbs due to having nothing to do.
It is true that things are unpredictable and you learn quickly that evening plans may not work out exactly as you hoped. But I think that is more a skill of learning the vibe of the department you're in and making it very clear that there are some engagements that you have to keep and the team will usually make do, especially if you give them notice. In over a year of my traineeship, I cannot think of a single time when I have told any of my supervisors that I had important plans one evening and they made me cancel them. The trick is to inform them in advance and make sure that they understand that this is important to you.