In fact, the mattress should not change, the body will tell you, if your body issued the following signal, it means that you should change the mattress:
Get up early in the morning;
Sleep time is getting shorter;
Lie in bed for a long time can not sleep;
Waking up in the middle of the night;
Skin pruritus;
Bed is not smooth, lying on the bed found a significant physical subsidence;
A little bit of movement can hear a clear squeak.
The above 7 signals as long as there are 1 kinds of you can consider changing the mattress, if there are more than 2 kinds of instructions to the mattress had to change the time.