It's good you realized you need to establish normalcy before venturing into the abnormal.
People leap off into the adventure so quickly at times that there is no comparison to be made to what life was like before the adventure. This is critical for pretending at any semblance of character development. It also puts a burden on the reader; if you leap into the story, you're relying on an audience so steeped in the tropes of your chosen genre that they can fingerpaint a background on their own or simply not care about it.
So, you've got the right mindset already, which is great!
How long it takes to establish normalcy is how long it takes. An exact thread or post count isn't needed. Do as much as you need, and no more. For strictly by-the-book writing, only include something if it furthers the plot or furthers characterization. Everything else is pointless. This will keep things interesting before you get to the meat and potatoes.
Example: if we need to know some important information about the protagonist's family, include it. Do not waste time describing the texture of the morning dew on the grass of the protagonist's lawn. Yes, it's nice and poetic, but who cares? If it has no importance to the plot of characters, or it's not going to crop up again, don't waste the reader's time going on about it.
You can ignore this rule if you have an unreliable narrator, but otherwise, only bring the reader's attention to things that are going to be important. If you have an unreliable narrator, then you may want to intentionally distract the reader by manipulating their attention.
Many times it's good to slow down and get into the description/worldbuilding, but you're correct in thinking that the beginning of the story is not the place to do that. Quickly sketch a colorful scene to set us firmly into your narrative, but move the plot forward.