In vanilla Minecraft, there is no way to tell exactly how much time a zombie villager has until it turns into a normal villager when you start the cure process.

This simple datapack will add a timer above the zombie villagers' heads when they are being cured so you can know how long they have left before they are cured.

HOW IT WORKS

When you cure a zombie villager using a golden apple, a timer will appear above its head to indicate how long it has left before it turns into a normal villager. It will also glow so you can know it is in the process of being cured when you go far from it.

In case you cannot see the timer above its head, or you are far away from it, the glow has a color; red indicates at least 1 minute, yellow between 10-60 seconds, and green below 10 seconds.

Once the timer hits zero, the display will disappear, the zombie villager will become a normal villager, and it will stop glowing.

Note: You can still use this datapack below 1.20.2, but every tick the timer display may sometimes display "Armor Stand". Using it at 1.20.2 and above will make the timer display more consistent (with the unnecessary "Armor Stand" display appearing less often).