If you want to go shopping in Bangkok, the best times are usually in the afternoon. Most shops don’t open before 10 or 11AM, and then they close around 9 or 10PM.
Of course, there always the 7eleven refuge, opened 24 hours on 356 days a year for the simple necessities of life.
Remember that it’s very hot in Thailand – but shopping malls are always well-equipped with air conditioning systems. That’s why if you plan to check out a lot of shopping malls, it might be a good idea to bring an extra shirt or thin jacket for the malls. This way, the temperature difference between the outside world and the acclimatized shopping mall world isn’t so harsh on you. Lots of people who aren’t used to that can catch a cold because of the steep temperature differences.