How often you may ask

Two floors, not one

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

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

The sensor's own floor. Aosong specifies "no less than 1 second" for the DHT11 and "greater than 2 seconds" for the DHT22. Go under it and the part either declines to answer or answers with a reading it has not finished taking.

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

So the practical floor is two seconds on both boards, and a DHT11 cannot in fact be read once a second through this library however much its datasheet allows it.

What that buys you

The cache is not a nuisance. It is the reason this works:

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

Two calls, one exchange. The first does the whole 24 or 6 millisecond conversation and stores all five bytes; the second finds the bytes still fresh and just decodes them again. Without the cache those two lines would be two separate readings, the second of which would fail because it came too soon.

What it costs you

It caches failures with the same enthusiasm. A read that timed out stores "failed", and every call for the next two seconds returns nan without trying again. One glitch therefore looks like a two-second outage, which is worth knowing before you go looking for a wiring fault that is not there.

And it makes a stale reading indistinguishable from a fresh one. There is no timestamp, no flag, nothing to ask. If your display updates ten times a second it is showing the same number twenty times over and the reader has no way to tell.

What to do instead

Put the reading on a timer and leave the loop alone:

unsigned long last = 0;

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

Two seconds is the right number for both sensors. For a room, ten or thirty is better still — nothing indoors changes measurably in two seconds, and the sensor is slower to catch up than that anyway.