Prado Museum is one of the most visited art museums in the world. It's a very popular tourist destination in all of Europe. The best time to visit Prado Museum is during weekdays when it opens at 10 AM. This will help you beat the crowd.
Another good time slot to plan your visit is between 2 PM and 4 PM. If you wish to visit the Prado Museum for free, you can choose Monday to Saturday between 6 PM to 8 PM and Sundays and public holidays between 5 PM to 7 PM. However, you should expect large crowds during the free hours.