I have some tables in the Oracle environment that I can get benefited from the new index. However, they have large tables ranging from 1M registers to 300m registers, so I first guess how long it will take to create an index, so I know the order of the least magnitude (hours
machine speed, memory, etc. There are actually many factors to consider in terms of which can affect creation time. Creation can have a significant effect on time.
What will I do, choose one of the big tables, make an index on it and see how much time it takes. Then, take it and Divide by the number of rows in the table and you expect that you should give some metrics for what you expect, again note that this will not be accurate, but this is just a rule of thumb. You can use it. This is very different because some tables have more columns, less sparse column values, but this is a starting point.
Ex Table X takes 3600 seconds to create an index, which has 3 million rows. So the metric is 3600 / 3,000,000 = 0.0012 seconds per line, so if Table W has 8 million rows, you can expect 0.0000 * 8,000,000 = 9600 seconds (or 160 minutes) to create an index. 
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