Mysore Zoo is open 8.30 in the morning till 5.30 in the evening. Also note zoo is closed on Tuesdays.
The most ideal time to visit Mysore Zoo is early part of the the say or during the evening. That's, when the day is not very hot.
This plan serve two purposes. First, exploring the Mysore zoo involves some considerable amount of walk in the open. Though the zoo is quite leafy and shaded, the 4km or so walk in the zoo during the mid day period could be quite overwhelming. Mornings and evenings are pleasant to walk.
Also during this hotter part of the day the animals tend to withdraw from the open and rest in the deeper part of enclosures, making spotting difficult.
Make a 2 hours or so itinerary early in the morning or later part of the day, say around 3pm.
Also note, though the Mysore Zoo timing is till 5.30pm, you need to reach early to cove the attractions at your leisure. For example, the entry to the aviary closes around 5pm.
The battery operated buggy may stop operating around the closing hours of the the zoo timings.
So, 8.30am to 10.30am and 3pm to 5.30pm are two ideal tour timings. Between the two, the afternoon plan seems more appealing as the animals start active as the evening sets in.
Plan your itinerary accordingly.