It ain't "ate", that's for sure.
Some evidence that it's two syllables, /ˈa.it/, comes from poetry, when ait appears in places where two syllables are needed to fit the meter. For example, this is from book VI of the Aeneid:
Ventum ad līmen erat, cum virgō "poscere fāta
tempus" ait; "deus, ecce deus!" Cui tālia fantī
ante forēs subitō nōn vultus, nōn color ūnus…
("They had come to the threshold when the virgin said 'It's time to summon fate. A god, behold, a god!' Suddenly, neither the face nor one color … of the one speaking this way in front of the doors …")
These lines are hexameter, the pattern of which requires that ait, in that sentence, be a two-syllable word with the first syllable "light" (a syllable ending in a short vowel, in this case ă). Rather than explaining hexameter in full, I'll just mark the breaks between feet with a slash:
Vent’‿ad / līmen‿e/rat, cum / virgō / "poscere / fāta
tempus"‿a/it; "deus,‿/ecce de/us!" Cui / tālia / fantī
ante fo/rēs subi/tō nōn / vultus, / nōn color‿/ūnus…
Each foot in hexameter must start with a heavy syllable (a syllable with either a long vowel or that ends in a consonant that doesn't begin the next syllable). There's one elided vowel, which I marked with an apostrophe, and I marked "conjoined syllables" with a tie. You should see that each foot starts with a heavy syllable, and one of those is the second syllable of ait.
As for how sloppily people actually said ait in everyday speech, though, I have no idea. Maybe someone more knowledgeable about colloquial classical Latin can provide some information about that.