"– How much time do they have? – Only eight minutes."
Translation:– Paljonko heillä on aikaa? – Vain kahdeksan minuuttia.
It's because it's uncountable. "Aika" would be used in nominative case when it refers to a specific moment in time, for example "nyt on aika mennä", meaning "it is time to go now". Using partitive case instead would change the meaning to "we now have time to go".