The phrase "burning the five p.m. candle" is an informal expression that means working late into the evening, especially on a Friday. It implies that someone is staying at work past the typical quitting time, often sacrificing their personal time. So, in this context, B is telling A to not stay too late at the office and to make sure they leave on time. It's a friendly reminder to avoid overworking, especially on a Friday when people typically want to relax and enjoy the weekend.

It's a play on the expression 'burning the midnight oil', I think (unless this is used in American English as a separate expression). 'Burning the midnight oil' means to work very late. Here, they wouldn't be working until midnight, but after five o' clock, if they stayed. So it means you don't want to be stuck working late on a Friday.