How long is the green led light supossed to flash?

[so33bama]

I had a sprint evo where the led light would flash indefinitely when you had a missed call,new e-mail and voicemail etc until you went in checked what it was flashing for.That phone's keyboard messed up so now I have a two week old evo.The light times out about 5 to 8 minutes.My question is why do some of the evo's time out and other don't.Any help on this matter would be appreciated.