Walmart stores are always busy, but they vary in their peak hours. Some stores are always busy while others are less busy. As a general rule of thumb, it's best to visit them before lunch, though this applies even better if you're shopping on weekdays. The later it is, the more likely that people are off from work and are looking to drop by and do some daily shopping. You're then more likely to get yourself stuck in a crowd of grocery shoppers.
If you want to contact customer service to get some insight on when they’re at their least busy hours, call them at 1-800-925-6278 or use their online contact form and leave a message.