How often you may ask

Two floors, not one

Drag the slider to whatever delay your loop has in it.

There are two limits, one on top of the other, and only one of them is the sensor.

The DHT11's own floor. The datasheet (§6) says the sampling period "should be no less than 1 second". Ask sooner and the part either does not answer or answers before it has finished measuring.

The library's window. MIN_INTERVAL in Adafruit's DHT.cpp is 2000 ms, for every sensor type. A read inside that window never reaches the wire: it returns the last result, decoded from the bytes already in memory.

So through this library the DHT11 is read at most once every two seconds, though its datasheet allows one.

What the window buys you

It is the reason this works:

float humidity = dht.readHumidity();
float celsius  = dht.readTemperature();

Two calls, one exchange. The first does the whole 24 ms conversation and stores the five bytes; the second decodes the same bytes again. Without the window those two lines would be two readings, and the second would come far sooner than the one second the DHT11 needs.

What it costs you

It stores failures just as readily. A read that timed out is stored as a failure, and every call for the next two seconds returns nan without trying again. One glitch looks like a two-second outage.

And an old reading looks exactly like a new one. There is no timestamp and no flag. A display redrawn ten times a second shows the same number twenty times over.

Put it on a timer

Leave the loop free and read on a clock:

unsigned long last = 0;

void loop() {
  if (millis() - last >= 2000) {
    last = millis();
    // one reading, here
  }
  // everything else keeps running
}

Two seconds is the shortest interval that gets a fresh reading every time. For a room, ten or thirty seconds is better: nothing indoors moves a whole degree in two, and the sensor is slower to catch up than that anyway.