I'd say 0-4 with colder being better. How long depends on space, time, patience, yeast and desired result.
48 hours should have some noticeable effect but you are encouraging yeast to become inactive and drop out due to gravity. You are also encouraging other particulate matter to drop out of solution. This takes time so however much time you want to give it. If using 1007 that takes ages to drop, I'd go minimum 3 weeks. Likewise for many Belgian yeasts. Something like 1272 or some British yeasts might get 24-48 hours.