How long does a quantitative finance worker work per day? Does he/she need to work over time? I've heard it depends on the field. Maybe no over work in big companies?
Quants typically do not work a set number of hours per day. Although average hours are from like 8 am to say 6 pm, people are usually expected to be flexible depending on many factors such as: need to take care of certain production systems pre-open or after hours, monitoring live strategies, need to complete certain projects within deadlines etc...
Also where you are in terms of developping a project can impact significantly your schedule. For instance if you are just begining building a new important pricing model or a new strat it is not uncommon to see people spend more than 12/14 hours per day to bring the project to live state as fast as possible.
On the other hand it can be flexible in the other direction too where it will be fairly easy to arrive late or leave early because your kids have a school event or even work from home certain days since many people have a vpn connection to access their office machine from work.
Overall how flexible or rigid your schedule will be is a function of how tied you are to production/operational processes but for the research/development it can be quite flexible from what i see either in banks or funds, as a pricing or algo-quant position.