Problem with millis()

I'm facing an error with millis() function. When the code starts everything works fine then after 1 hour the millis function fails.

Below is my code

long cur_millis,prev_millis;

cur_millis=millis();
prev_millis=millis();
while(cur_millis-prev_millis<30000)
{
     take_photo();
     digitalWrite(25,HIGH);
     upload_photo();
     cur_millis=millis();
}

digitalWrite(25,LOW);

In this code the while condition becomes always true after 1 hour and working as infinite loop.

Please help me fix this problem.