Temperature won't really matter. You just didn't have enough agitation! All you need to do is shake the jar for a few minutes, and you'll have your butter.

Do it in a pint or quart jar at home, and you'll get a feel for how long it takes. It will take longer if the cream is room temp, but not horribly so. Whether or not the cream is pasturized makes a difference, too. Sometimes that can make it take longer to separate.

On average, with refridgerated heavy raw cream, it takes about 15 minutes of constant (but not necessarily vigorous) shaking to turn 2C of cream into 1/4c of butter in a quart jar.