It's easier to tell you the busiest times. Lunch time around 12pm - 2pm, after work time around 3:30pm-6pm, Saturdays all day if it's a popular parlour.
From my experience if they open Sunday late afternoon is usually quiet as most people are with their families or out, as mentioned by bigboy96 morning, but not first thing before lunch and if it opens late around 7 pm-ish is usually quiet after tea time.