In general, it makes sense to do timed sets once you're achieving high accuracy in a topic untimed, or flexibly timed.
At the same time, you can work on reducing your time per question at any point in your prep.
For instance, when you're working on Quant topic, you can note how long it's taking you to answer questions and look for ways to shave the time per question down, such as by taking fewer steps, minimizing the number of variables you use in translating a word problem, or using smart numbers.
Similarly, in Verbal and Data Insights, you can look for ways to be more efficient in answering questions. In Verbal, learning to quickly identify what's most essential about each answer choice can help you speed up. In Data Insights, learning to note key constraints or where information lies in a prompt can help you find the information you need more quickly.
Also, you could use the following rating system:
Level 1 - You knew basically how to answer a question you saw.
Level 2 - You got the question correct.
Level 3 - You got the question correct in the amount of time allotted for such questions on the test.
You can shoot for as many Level 3's as possible without cutting yourself off in answering any question.