MySQL performance of applying unique constraint single column vs combination column
The duration of the creation of the index will be directly related to the size of the index. I don't think it would be wildly different for the 2 options provided. Under the hood MySQL is going to copy the table to recreate it in the new structure, so consider the amount of time MySQL will take to read the data and write the data. The process is single threaded too. If your storage is slow this will take some time.

Tips:
- Ensure that fast_index_creation is enabled
- Group multiple ALTERs 
- use pt-online-schema-change tool to perform it online. This will take longer but will permit you to continue to use the table.