I have some experience in using acetone/dry ice to bring about a temperature of nearly -78 oC . Exact amounts were not needed; I used to add little acetone slowly to the dry ice every now & then to maintain that temperature for about half an hour.
I know that ethanol/dry ice gives a temperature about -72 oC & I think that varied additions of small amounts of alcohol will work but do not exceed 1:1 ratio of ethanol to dry ice.
I think that you're question is incomplete. What kind of container are you using to hold the slush? What are the environmental conditions? Normally, a small dewar will hold a temperature for quite a bit longer than 10 minutes. But often a slush is made ad hoc and you get a feeling for how long it will hold a temperature with experience. If you want real numbers, you probably will have to test your setup under your conditions.