It takes as long as it takes, like in any market, sometimes assets can remain incorrectly priced for years. That's why sometimes companies that are way undervalued are outright taken over, in order to "release" and "free up" that latent value. Of course, the easier it is to "arb" out the pricing misalignment, the faster prices should fall back in line (ie. keep dumping all the SBD you can get your hands on in exchange for BitUSD until the spread tightens).

As for why it's better not to power up in such cases, this thread on one of my past posts may help further clarify the topic for you...

Link: DON'T POWER UP YOUR POSTS WHEN SBD IS WORTH MORE THAN USD