Today, I wanted to discuss a role I had as a research assistant at a university. I had discussed it using language like "During my research assistancy, I did xyz" or "I had this experience in my research assistancy." Compare to the equivalent sentences with "PhD candidacy" instead, which I hear from time to time.
When I tried to use assistancy in a later email, it was underlined red. To my surprise, many dictionaries don't have that word, including Merriam-Webster, dictionary.com, the Cambridge Dictionary, and the Oxford Learner's Dictionaries. Spellchecker.net even lists it as a spelling mistake.
The Google Books Ngram Viewer does list usages of it, but only barely, and the word seems to have fallen out of (relative) popularity 100 years ago.
Given this information, can "assistancy" be considered a word? For reference, the similar term candidacy means:
the state of being a candidate
If assistancy is indeed unacceptable, what word means "the state of being an assistant"?