Is "CREATE INDEX` in MySQL a Linear Operation? What I mean is the following:

If creating an index on a table with n rows takes t time. Will creating an index on the same table with 1000*ntake approximately 1000*t time.

What I'm trying to achieve is to to estimate the time it takes to create the index on the production database by creating the same index on the much smaller test database.
Index creation is essentially a sort operation, so is at best has a growth complexity of the order n log n on average (you might find it does better in some cases, and is not likely to do much worse).

If all your relevant data pages fit into RAM and are already in RAM, and the index will fit also, and your DBMS does not force index pages to be written before the creation is complete (so index blocks are not updated on disk multiple times during the operation), then the speed of writing the resulting index to disk will be more significant than the time taken to perform the sort - so you might find you get closer to a linear relationship between number of rows and the time the index creation takes - but if you assume the worse case you are less likely to be unpleasantly surprised!

Remember that unless you are not going to stop access to the production database during the operation any index create will be competing for IO bandwidth and/or locks with other activity, so you should try to account for this if you are doing your timing estimation tests on another system even if it is identically configured.