Trigger countdown with 433MHz transmission
You say The full code includes some differentiation mechanism for all sensors so I'll assume there's some function GetSensorId() that returns 0 to NUM_SENSORS - 1.

So, you need to keep track of a) how many sensors have reported in and b) time since the last report. You can't just count how many sensors have reported in since there may be a chance one could report in more than once. So I would use an array:

// Assume you never receive 0 from sensor.
const int NO_DATA = 0;
const int NUM_SENSORS = 4;
int sensor_data[NUM_SENSORS];

void setup() {
    // Initialize array
    for (int i = 0; i &lt; NUM_SENSORS; i++) {
        sensor_data[i] = NO_DATA;
    }
}


Also, a variable to keep track of time of last message:

uint32_t last_message_time = 0;


So, in loop():

void loop() {
    // Check for data
    if (mySwitch.available()) {
        int value = mySwitch.getReceivedValue();
        if (value == 0) {
            lcd.clear();
            Serial.print("Unknown encoding");
        } 
        else {
           // Determine which sensor sent the data
           int sensor_id = GetSensorId();
           Serial.print(value);
           Serial.print("%");
           // Save the data and the time of the message
           sensor_data[id] = value;
           last_message_time = millis();
       }
    }
    // See if all sensors reported
    byte sensors_reported = 0;
    for (int i = 0; i &lt; NUM_SENSORS; i++) {
        if (NO_DATA != sensor_data[i]) {
            sensors_reported += 1;
        }
    }

    // When 0 sensors have reported in, do nothing
    if (0 != sensors_reported) {
        // Otherwise, at least one sensor
        // check the time
        uint32_t elapsed = millis() - last_message_time;

        // If have all data or time's up, do something
        if (NUM_SENSORS == sensors_reported || elapsed &gt; SENSOR_TIMEOUT) {
            // Do something

            // Reset state
            for (int i = 0; i &lt; NUM_SENSORS; i++) {
                sensor_data[i] = NO_DATA;
            }
        }
    }
}