Being an intern, I say there is a pretty good work-life balance. However, it depends largely on the client you are assigned. I worked normal 8 hour days, never having to stay late. However, I know others who were helping with larger clients with a tight deadline, and therefore worked more overtime. But overall, as an intern, it is usually typical that you don't work much overtime, if any, so you have plenty of time in the evenings to enjoy.
Expect a lot of work life balance because interns are capped at 40 hours a week. So you have time to hang out with other interns, explore the city, and enjoy social life. There were times where I knew I wouldn't get paid to do over-time work, but I still turned on my laptop at home anyways because I knew I was leaving and moving on to my next client and I didn't want to leave with unfinished assignments/ tasks. But that was only for 2-3 nights throughout my entire internship.
It depends on the type of internship you have. I have been fortunate enough to complete both a winter/spring internship and a summer one. Most clients have a 12/31 year end and audit opinions are normally due in March an April of the following year. On average auditors are working extremely hard from January to April, so winter/spring internships are a much serious matter. I will be completely honest and depending on which engagement team you are placed on there is not too much work life-balance. On average you will work about 55 hours a week minimum during the winter/spring internships, however summer internships are much more mild and there is more room for work-life balance. Summer internships are more relaxed because audit opinions have been issued and it is now back to the planning stage of the audit. During this time, employees often take vacation days because there is less work to be done. With a summer internship you will expect a normal 40 hour work week.