If you're asking because you're working against a deadline right now, here are a few expert-backed strategies:
: Stick to "until" for your academic work. If you must use a shortened form in a creative or informal context, use "till" or the archaic "'til"—though the latter is rarely seen in modern formal writing.
: Remember that "till" is also a verb meaning to prepare land for crops (e.g., "to till the soil"). Use it this way if your essay is about agriculture.
: Always leave at least five minutes at the end for a final Grammarly or grammar check to catch simple errors.
: Maintaining a formal tone throughout is key. Avoiding contractions and informal prepositions like "till" helps ensure your academic argument is taken seriously. Finishing Your Essay on Time