Which word best describes a large amount of time spent studying vocabulary?

The term "substantial" accurately conveys the idea of a large amount of time spent studying vocabulary. When something is described as substantial, it indicates that it is large, important, or significant in degree or quantity. In the context of studying vocabulary, spending a substantial amount of time implies a dedicated and noteworthy effort, which contributes meaningfully to one's language learning.

This term highlights the importance and impact of the time invested in vocabulary acquisition. It suggests that the effort made is not just a trivial or small endeavor, but rather one that has the potential to yield significant results in mastering new words and improving language skills.