In reply to TomAlford:
Every time you fall or sit on a boulder mat it slightly damages the foam, it will gradually become soft over time. The speed of deterioration will be dependant on use, quality and type of foam etc.
Falls in general are of a high force, but a short duration.
Sitting or sleeping is a low force, but of a long duration.
There are also usually 2 types of foam in a boulder mat. These have an elastic and viscous effect, as such they react and degrade according to different types of force.
I sit and sleep on mine and their lifespan, is still measured in years, it could have been longer without misuse, after all they are not cheap, but then I do not care enough.
Some climbing centres use them as seats for customers. You could ask them how long they last, but they do seem to use the very firm closed cell foam in these cases.