Two to four weeks for the performance effects to become noticeable. The mechanism is gradual saturation of muscle stores, and that takes time.
There's no immediate hit with creatine. It's not a stimulant. You won't feel it in the first session. Some men take it for a week, feel nothing, and stop. That's the most common reason it gets abandoned too early.
The loading question
Some protocols use a loading phase: 20g per day for five to seven days, then dropping to 3 to 5g daily to maintain. This saturates stores faster and means you notice effects within the first week.
The alternative is just starting at the maintenance dose of 3 to 5g daily and letting it build naturally over three to four weeks. Both approaches end up at the same place. The loading phase just gets you there faster. It can cause digestive discomfort for some men, so if your gut is sensitive, skip it.
What you should actually feel
Not a dramatic shift. What most men notice is that training feels slightly more manageable. The last two reps of a set feel less like a wall. Recovery between sets is a bit faster. You're moving more weight without it feeling heroic.
Over weeks, that adds up. More volume done at higher intensity equals more adaptation. The supplement doesn't build the muscle. It lets you do the training that builds the muscle.
One thing to know
You need to stay hydrated. Creatine draws water into muscle cells. If you're under-hydrated, some men get headaches. Drink more water than you think you need, particularly in the first few weeks.
Consistent daily use. That's it.
Related reading: Is creatine worth taking if you're not a bodybuilder?, Does creatine affect testosterone?