Problem by controlling application with Arduino!

I want to read in every ten minutes one sensor and sending a character which print these values to an C# application...The command which accepts from serialport is serial.read();
The controller stucks here and i don't want something like that.I want to continues read...When the user didn't send command to retrieve values the Arduin's software should continues reading in every 10 minutes.

    if( millis() >= (10UL*60UL*1000UL) && (!isPressed))
    {
       //read values again if 10 minutes has completed
      //and the user didn't press 'l'
    }
    else if( isPressed ){
      byte serialData = Serial.read();     
      if ((char)serialData =='l')  {      
       //sent data to C# app 
      } 
      else {
      //something else      
      }