Thunderstone armor stores whatever haste it has been given, and it won't run out.

For example, while wearing thunderstone armor and being given haste 15, you will permanently have haste 15 until taking the armor off. Upon putting it back on, you will then have haste 15 once more. Relog doesn't solve it, but server restart reverts it to the base haste until being given a higher level of haste once more.