They put your application into the Gonkulator!! That spits out a score very quickly.

Then you wait to hear from them based on where your score is relative to other applicants. When they send out interview invites they do so to the number they feel they need to interview and they chose those that have the highest score according to the Gonkulator! As they send out more invites the score needed to get an invite moves lower.

When you update your app with additional qualifications or flight hours the Gonkulator calculates your new score.

Once your score is higher than the score needed to get an invite, (either through the required score going down due to additional invites or your score going up due to updates) you will get an invite!

Time since you applied has literally zero to do with it.

ps. While the process the gonkulator goes through to calculate a score is generally understood, (more ratings, more PIC time, more turbine, more, more, more is better) the specifics of exactly what counts for how much is a closely gaurded secret