"Two years is too much time."
Translation:שנתיים זה יותר מדי זמן.
Could I also use a copula here instead of זה? For example, would שנתיים הם יותר מדי זמן work?
Yes, it works (except it's שנתיים הן יותר מדי זמן, as שנה is feminine).
I used הן which was accepted so it appears that the זה is incorrect and should be זאת
The copulas זה/זאת/אלה usually match the second noun in "<noun> is <noun>" type of sentences. זמן is masculine so זה is right answer.
I was talking about "z-copula", i.e. זה/זאת/אלה. Also, what I wrote is not 100% accurate, and many times זה is used even when both nouns are not singular masculine. I'm not sure if there are strict rules.