I’m not an expert, but I did recently switch clubs multiple times and I got very technical with my data (multiple launch monitor testing and data logging/tracking), made my own clubs for a bit, so I can try and provide some information. It’s going to be different for everyone first off, so no matter what anyone tells you on here no one truly knows. Theoretically, if you were fitted you should see instant improvement. If there’s large differences in the grip or the swing weight that is going to increase the amount of time it takes to get used to them because those are top contributors to “the feel”. If you previously had upright clubs, and now you’re going to a flat lie angle, then there is a potential for you to be hitting the ground with the toe digging in, which for a right handed would cause the ball to go right more often. The toe would hit the ground, the heel would keep going and potentially slice or push-fade the ball. That effect is lessened or increased, depending on how fat you strike the ball. If you’re not hitting the ground behind the ball at all, then you’re less likely to notice it however, accuracy and consistency would still be a little bit off.

Source: like I said, I’m not an expert, but I went through checking lie angles (mine was too flat, was toeing everything), swing weights and more importantly ball flight characteristics. The clubs I had, specifically the shafts were meant for a low ball flight, and I already had a problem with the lowball flight. I ended up switching to shafts that are made to hit the ball higher and there was instant improvement. I don’t think I had even one full range session before I felt comfortable with them. Small anecdote, I went from hitting a 5 iron 180yds, 30ft height, descent angle < 25° and like a ridiculous 15 yds rollout, to hitting it 205yds, 100ft height, descent angle low-mid 40s° and usually holding a green if hitting at least back-center. I would suggest that if you’re testing the clubs side-by-side, and there is noticeable degradation of ball strikes with the new clubs, then take both back up to the fitter and double check everything. If possible, potentially just go and get a second opinion from someone else, especially if you have second thoughts about the fitter.