A long period or duration (a long dozen could imply a lengthy time involving twelve of something)

लंबा समय

English Usage: It felt like a long dozen before we reached the finish line.

Hindi Usage: हमें फ़िनिश लाइन तक पहुँचने में लंबा समय लगा।

A set of twelve items (as in a dozen eggs)

एक दर्जन

English Usage: I bought a long dozen of eggs from the market.

Hindi Usage: मैंने बाजार से एक लंबा दर्जन अंडे खरीदे।