I recently started playing starcraft II and was originally placed in Silver League. After playing various matches winning half and losing half or so I got demoted to Bronze League, personally at the time I didn't mind because I wanted to learn the game the right way, etc. However, I only played 6 or so games before being demoted, I have been in Bronze league for a month or so now and have maintained a rank 3 or so during this period and I constantly play with Silver players, etc,(and won) and I have still not been promoted. I had a winning streak of 10 matches and no promotions, yet I was demoted in the first 10 games.
The most comprehansive guide about SC2 ladder is here (I belive)
As for your question "when will I be promoted": just keep playing and you'll get it. Especially if you constantly see opponents from higher league.
By outperforming the rest of your league, it is possible to get promoted into a higher league. If you are in Bronze but playing against Gold players, you would expect to be promoted to Gold, but that doesn't always happen immediately.
This is because the system requires a certain degree of confidence before players can be moved to a new league, otherwise they would bounce around from league to league too frequently for leagues to be meaningful.
FYI: Blizzard support will not answer you if you call them about ladder "issue".
The exact mechanics of league placement are a secret held eagerly by Blizzard. However, folks have guessed parts of it rather accurately. You can read about what we know on Liquipedia.