Yeah, everyone is different. I tried experimenting with 20-30-40 minute intervals and decided 25 is the optimal for me. But it really has to be distraction free. I use brain.fm or instrumental music (no lyrics) or I just keep my headphones on so nobody bothers me. It doesn't sound very challenging, but after five pomodoros I truly can feel it, after eight - I am mentally exhausted. Ten pomodoros mean I either cheated or something else.