We normally get there at around that time or just after. Two years ago it was a bad idea, there was a big queue at the box office. Last year was fine though. They'd moved the box office, made it much larger with more staff and there was no queue at all. Assuming it's like last year, 10 should be fine. The only thing to be wary of, is that it can take a while for them to open the main festival site. Last year we were kicking our heels in camping for a bit because it took them a while to let people in. I think maybe it was around 2pm? Might be misremembering though.