The instant pudding takes between 5 to 10 minutes to set when you put it in the fridge.
As long as you've made the mixture of the instant pudding correctly the instant pudding should begin to set within 5 minutes but it may still be a bit thin.
However the longer the instant pudding sets in the fridge the more it thickens so if it's not thick enough after 5 minutes keep it in the refrigerator another 5 or 10 minutes and it should be ready.
When making instant pudding you must put it in the refrigerator for it to set otherwise when left out at room temperature it will not set.
Instant pudding does take a bit of time to make and set and so it's not really instant but it's easier to make instant pudding instead of making instant pudding from scratch.