There are too many variables that affect that to put a hard fast number on it. Acidity, carbonation, and a few other things all have an impact on how sweet someone perceives the cider to be. For reference though, the list below is from the 2015 BJCP Cider Guidelines:
- Dry: below 0.4% residual sugar. This corresponds to a final specific gravity less than 1.002. There is no perception of sweetness.
- Medium-dry: 0.4-0.9% residual sugar. This corresponds to a final specific gravity of 1.002- 1.004. There is a hint of sweetness but the cider is still perceived primarily as dry. Also known as semi-dry.
- Medium: in the range between dry and sweet, 0.9-2.0% residual sugar, final gravity 1.004- 1.009. Sweetness is now a notable component of the overall character.
- Medium-sweet: 2.0-4.0% residual sugar, final gravity 1.009-1.019. The cider is sweet but still refreshing. Also known as semi-sweet.
- Sweet: above 4.0% residual sugar, roughly equivalent to a final gravity of over 1.019. The cider has the character of a dessert wine.