It depends on how you use your practice time, what practice techniques you use, and how effective/efficient you are. And also how well you learn music (one of my piano teacher's "adopted kids" is honestly the most freaky kids in the world. She's quasi-professional now, but she has some sort of savant-like musical gift, save for the fact that she's not autistic, and is actually very normal when she's away from the piano). But seeing that not everyone is a freaky genius...
I had to do my grade 10 twice because of 2 reasons: first, I have really bad exam/stage-fright, and when I play for other people, my hands shake, and I play everything really fast and out of control, and I lose my concentration, and I'll make every mistake that I ever made, plus some new ones. Secondly, because I didn't know all my pieces inside and out to prevent my exam-fright from getting in my way.
My practice times are really inconsistent. One day, I might play 3 hours, and another day, I might do as little as 15 minutes or none at all. I try to keep my no-playing days at a minimum, but you know, it's easy to get distracted and pulled into other responsibilities, and to be honest, my schoolwork comes way before my music. If you're consistently doing one hour and you set well-formulated goals in your mind before you start, and accomplish them by the time you're finished, then you're well on your way. (Mind you, your goals shouldn't be "play through each piece twice"). Right now, getting ready for festival, I'll have goals like "get 2 pages of that ugly Canadian piece to fluency, or memorise 2 pages of the Bach."
I get worried that you didn't mention your technique requirements... That was my first downfall. Those arpeggios are supposed to go pretty fast. I got all of my technique to at least 10 bpm faster than they requested, that way I knew that they were secure.
In terms of concentration, experiment with your situation right now. What is it that makes you lose focus? This year, when my sister moved out, I found that I could concentrate much better, and one day, my friend phoned with an emergency during my normal practice hours, and I was forced to practice at 9-11pm, and I found that I felt much more at ease practicing in a dark room and I wasn't as easily distracted. But that differs for each person. I also found that while I practice well at night, if I'm doing anything really intellectual that requires grounding something deeply in my memory, it's better to do it in the morning on a day off. (Like memorizing aleatoric music, usually of the Canadian or Hungarian variety). So just move things around until they feel right. Also, having a detailed goal in mind really, really helps because you force yourself not to leave the piano until it's done.
Topic: Practicing