As fast as one day. There is no minimum number of trading days in the challenge phase. If you hit the profit target on your first day while respecting all the rules, you pass.
Is there a consistency rule in the challenge phase?
No. The challenge phase has no consistency rule. You can make your profit in a single day or spread it over many days. Both are fine.
Note: a 40% consistency rule does apply later, in the funded phase. See the funded phase section for details.