Cached services: upper limit for "Cache expire" value?

Hi all,
I experienced a strange problem: if I set the “Cache expire” value to 15000000 (a really really big value, I know) the output values are not cached. Instead, if I set a value of 10000 (around a weak, if my math doesn’t fake) everything works fine.

I find no evidence of an upper limit for that value. Did someone experienced such behaviour?