"at all time" or "at all times"?

"She's cheerful at all times." Also can say "She's cheerful all of the time."

The key here is countability. Time is countable in terms of number of times, such as bottles. Time is also something that can be uncountable (some of the time, most of the time, all of the time), such as water.

This is similar to banana/bananas. Bananas are countable (all bananas were eaten). But also banana can be uncountable, that is the substance itself "in" the banana (all of the banana was eaten).