There are two things to keep in mind before posting your photos to Instagram: the time zone of your followers and what time they are often checking Instagram. Most of the Instagram users login in the morning and in the evening.
For the brand, the perfect time to post is at midnight as they get about 4 hours of life before being lost to a crowd of photos on Instagram.
Post at the time of day when you feel that your followers have free time and are checking their account. For example, if you are a college student, post your photos in the afternoon, when the afternoon break occurs. Or, if you are in a city like Mumbai, where people check their social networks in the morning and Shyam in local.