First, let's take a look at the difference between the Bronze Age and Medieval.
Both actually cover a lot of ground, this will depend on where you are starting. Late Medieval saw the invention of gunpowder for instance, but for the majority of the era, it was not an advancement.
Wikipedia has a decent list, although some are later and more towards Renaissance, but keep in mind that though many of these advancements were discovered and used, they were not all common for the majority of people.
Here are some high points that WERE common:
- agriculture, specifically, crop rotation
- founding of Universities
- Buttons. You know, the things on clothes.
- the idea of quarantine
- the wheelbarrow. (although wheeled carts are bronze age, this one wheeled model is actually considered a medieval invention)
- the chimney.
Later and less common, but still very important to development:
- zero, as a concept. Arabic numbers were all the rage with monks, but...were outlawed in many places for centuries, as far as marking common goods, until the banking system of the Renaissance made it necessary, and the invention of the printing press made it less confusing.
- advances in astronomy. with instruments to measure, some of which re-discovers bronze age discoveries, some of which is new.
You can take a look at the list for more. Many of these concepts, once learned (like the wheelbarrow and the button, as we know it) might not actually go away.
You should also consider that while parts of the world were firmly Medieval in the 12th century, there were other parts of the world just entering the Bronze Age. And, in places like Russia, when other countries were firmly in the grip of the Renaissance, most people lived a more Medieval life in that country, with Medieval social mores and tech.
You're defining the era based on what Europe was doing, but even in Europe, there are always places ahead and behind on specific concepts, or as a whole.
The Bronze Age, besides the obvious metalworking stuff, gave us things such as:
- domestication of animals, including horses
- the wheeled cart
- governmental systems
- the socketed axe
- tools and weapons of all kinds.
- astronomy, depending on when and where.
You might want to investigate WHEN in the bronze age you want these folks knocked back to, because this goes from 3300 BC to 1200 BC, depending on who you ask and where you're looking at on a map.
So how long will it take for your people to get re-Medieval?
I think there are too many factors involved for me to give you a specific number. You might not think this is broad, but on the wide, wide scale of the history of these two eras, it really is. I CAN answer that how long it will take will depend on these factors:
- population, before and after. Look at population estimates for the Bronze Age, in the areas you know and look at the population during Medieval. This is going to be WIDE in both eras. Cannot stress what a huge impact this has. If there are too few people, advancement will not happen as quickly. You can actually almost solely base the answer to this on breeding on how many people there are and in what concentration. This will be interconnected with farming.
- Know when you are starting and how fall they are falling. Tech available during early Bronze is different then late, same for Medieval.
- Some things won't be lost. Like buttons and wheelbarrows, but depending, farming techniques might be, and mills definitely will be.