Yes
Typically, yes. The newer the release the more previews there will be. Some Marvel movies had nearly 30 min of trailers. In some more rarer instances (think limited release indie films) there might only be 10 or so min of previews.
About 15 minutes...
Easily count on 10 to 15 minutes for previews
Most movies there will have about 15-20 minutes of trailers, which begin at the start time of whichever movie you're going to see.
It really depends on how many previews and such they have to show. I've been in movies to where it started exactly on time as posted and others where it started 15min after posted time. For planning purposes, I'd suggest you expect at least a 5min delay for previews.
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