How to get the overall average of the data received
void setup(){
Serial.begin(9600);
}
void loop(){
    static unsigned long count=0;
    static float sum=0;
    float ave=0;
    int volt=analogRead(A0);
    float voltage = map(volt,0,1023,0,2500) / 100.0;
    Serial.print("Voltage: ");
    Serial.print(voltage,2);
    sum=sum+voltage;
    count++;
    ave=sum/count;
    Serial.print("  Average: ");
    Serial.println(ave,2);
    delay(3000);
}


This runs quite a while. The average is restarted only with a Reset of your arduino.

Alternatively, you might want to learn about a moving average (aka low pass filter)