Without "they would," the meaning is unclear because the sentence would seem to say that the time the plastics take to dissolve was simply longer than it originally seemed. In other words, the measurement of the time needed to dissolve was somehow clarified and found to be longer than the measurement first appeared to be. The actual meaning of the sentence is that there was some prior expectation that the plastics would take less time to dissolve.

You might also try asking yourself, "something took more time than WHAT originally seemed?" Was it simply that the measurement of the time was greater than it originally appeared to be or was "the time they would take to dissolve" more than originally expected. The intended meaning is the latter.

Note also that the verb "take" is left off the end of the sentence. It is implicit.

Hope this helps!

Also, the pronoun "it" is really a placeholder pronoun (AKA expletive pronoun) in this sentence. The pronoun "it" does not refer to "time." Without "they would," one might think that "it" refers to "time."

well i fail to explain the reasoning , why wont it be QA.A when its perfectly right and straight forward...
Please advice