Hold button more than 5 seconds, do something. Arduino - button

Question

I have a hobby project that sends a string "1" or a string "0" to my webserver. My arduino work as a client, and It works, but now I want to add another statement.
If the button is held down 5 sec or longer, send string "5" to my webserver. I do not know how I can get the button to count the seconds I have hold the button. Can you please help?
Here is the code:
#include <SPI.h>
#include <WiFi.h>
byte mac[] = { 0xDE, 0xFD, 0xBE, 0xEF, 0xFE, 0xED }; // MAC adresse
char ssid[] = "test";
char pass[] = "123456789";
IPAddress ip(192, 168, 0, 143); // Klient IP
IPAddress server(192,168,0,100); // Server IP
int port = 2056;
boolean btnpressed = false;
int status = WL_IDLE_STATUS;
WiFiClient client;
unsigned long lastConnectionTime = 0;
boolean lastConnected = false;
const unsigned long postingInterval = 1000;
void setup() {
attachInterrupt(0, AlarmPressed, RISING);
// Opne serial port
Serial.begin(9600);
while (!Serial) {
;
}
while ( status != WL_CONNECTED) {
Serial.print(" SSID: ");
Serial.println(ssid);
status = WiFi.begin(ssid, pass);
delay(1000); //
printWifiStatus();
Serial.println("Connecting to server");
}
}
void loop() {
if (client.available()) {
char c = client.read();
Serial.print(c);
client.stop();
}
if (!client.connected() && lastConnected) {
Serial.println("Disconnecting.");
Serial.println();
client.stop();
}
if(!client.connected() && (millis() - lastConnectionTime > postingInterval)) {
SendData();
}
lastConnected = client.connected();
} // Slutten paa loop
void AlarmPressed() {
btnpressed = true;
}
void SendData() {
// if there's a successful connection:
if (client.connect(server, port)) {
Serial.println("Connecting...");
// Send data til server:
if (btnpressed == false){
client.write("0");
Serial.print("No Alarm");
Serial.println();
btnpressed = false;
}
else {
client.write("1");
Serial.print("Alarm !");
Serial.println();
}
// note the time that the connection was made:
lastConnectionTime = millis();
}
else {
// if you couldn't make a connection:
Serial.println("Connection failed");
Serial.println("Disconnecting.");
Serial.println();
client.stop();
}
}
void printWifiStatus() {
Serial.print(" SSID: ");
Serial.println(WiFi.SSID());
// print your WiFi shield's IP address:
Serial.print("IP Address: ");
Serial.println(ip);
// print the received signal strength:
long rssi = WiFi.RSSI();
Serial.print("Signalstyrke til forbindelsen (RSSI):");
Serial.print(rssi);
Serial.println(" dBm");
}