There is no general time limit.. it all depends on the quality of your portfolio and your Principal. If they are confident, they could send you for cluster interview at the next available opportunity...

So the key question is ... are they preparing you for the role.. that is the question.. if yes.. then build up your portfolio fast.. include everything significant things that you have done in the past.. if there is nothing much.. then take a year or 2 to build up.. while portfolio is one thing.. you must learn to think like an ST.. go and read up on some literature.. identify a pet focus area related to your discipline that you want to focus to strengthen T&L.

hope i address your query.