I don't share the assumption that Duolingo should avoid colloquialisms. Fluency in a language involves mastery of both formal and informal speech.
It's true that "till" is less frequent in formal contexts (at least in contemporary American English). But why do we view it as less formal?
The reply I hear most frequently from students is that "till" is a misspelling of " 'til" which is an abbreviation of "until".
But actually "til" existed in Old English and is found in an inscription from around 800 CE. (In the following centuries, "till", "tyl", "tylle", "tille", "tylle" are variant spellings -- standardized spelling is a very modern invention.) "Until" ("vntil", "unntill", "vntill", "untell", "untyll", etc.) doesn't appear until the 14th century and is derived from "till" by the addition of a prefix that descended from Old Norse "und".
The case of "till" is interesting because it illustrates how the attitudes that we have toward words are often based on false beliefs about their origin (and the assumption that the original form is the "correct" form).