[Suicide Junkie]

The stockpile is limited by the number of storage facilities you build, plus 50k as a starting amount. Everything above that spoils away.

Note:
Storage facilities do not need a spaceport.

Although you do need some storage facilities to handle massive retrofits and other fluctuations in usage rates, each storage facility is taking up a slot which could be producing minerals, research or other valuable things.

PS:
To see your current limits, look on the budget screen at the very bottom.