okay, so indexing is just a fancy way of saying that it is building a database, or in a more lamens terms, a card catalog (similar to what libraries used to have).
So, the amount of time it take scan vary greatly depedning on a few factors.
The biggest factor is HOW MUCH IS THERE YO INDEX, and in your case (only 25 emails) this is a small amount, so it should be quick (less than 10 seconds)
The second bigest factor is the computers ABILITY to process an build the database. That is determined by the speed of the CPU, and the amount of RAM, and etc etc. The thrid biggest is the current background processes. If you are already doing something that is taxing the CPU, and because an index is always a secondary priority, and will take longer if you already have something running.
All in all 25 emails to index.......not long.
But it depends on WHAT is doing the indexing?
Is it Google? Or is it your computer?