Naturally, having more time to ponder each move lets skilled players figure out better decisions and avoid errors one might make when there is a pressing time limit, with more time allowing more informed moves.
However, we're also just human, and even the smartest chess players have imperfect memory and ability to envision deeper and deeper decision trees, so at some point the benefits of additional time would become negligible. In general, how much time per move would be sufficient such that increasing it further would not result in a noticeable improvement at the grandmaster level?
I'd like to focus on just the human computation aspect, so you can ignore fatigue or attention-span effects from spending many hours on a single chess game.