The airspaces you are required to establish 2 way communication is A, B, C and D. Airports inside B, C and D have operating control towers and therefore require you establish the communication. When the control towers are not operating (some close after hours), then the airport and airspace around it becomes class E and you follow those rules. The AFD will detail the hours of operation. Many class D airports and some class C ones will close in the later hours until early morning.
To enter class B, however, you must hear the actual magic words "you are cleared into class Bravo". With C and D, you only need to hear you call sign acknowledged ("Cessna N4752....."). If you only hear the words "standby", then that does not constitute 2 way communication even though they acknowledged you.
Class E exists to provide instrument procedures to the airports underlying them, none of which have an operating control tower. If you are flying VFR, as pointed out in the prior thread, there is no FAA regulation that you *must* establish 2 way communication.
It is, however, a very good practice and your obligation to communicate your position, type of aircraft and intentions (e.g. "Redbird traffic, Cherokee 5 miles Southeast inbound. Will make left downwind for landing. Redbird traffic") And then make calls as you enter the legs for landing, and even let the UNICOM listeners know when you have taxied cleared the runway.
On a busy nice sunny day, expect to hear others doing the same. On a quiet boring night, you might hear nothing. But you're still announcing yourself in case there is other traffic in the area, or on the ground preparing to take off.
