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With immigration clearance, it takes as long as it takes. Newark is often used as a Port of Entry by many non US citizens flying into the US. There are just too many factors involved. If it helps, from what I know about non US citizens arriving in extremely busy POEs such as JFK- NY, Dallas and Atlanta, usually, regular tourists will clear immigration in about an hour.
If you are on a single leg ticket, and if you miss your flight, you will be put on to the next available flight. In the US, mostly, if someone misses a domestic flight they are given an opportunity to fly in the next available flight if seats are available.