If you are using 7.1.8, it should be an Elastic Search index (which uses Lucene underneath).
The size of the index grows the number of cases, rules or data instances grows in your database.
Could you please let us know what the current index size is?
Also, the time taken to index is dependent on the number of cases, rules and data and the size of each instance. You can roughly approximate the size of an instance by doing a length on the BLOB column.
Note that Elastic Search indices (Lucene indices) grow and shrink as indexing updates the index with entries in it. This results in the index actually consuming up to 3 times the size of the index if it was freshly built.