Seconds
Hermione wound back the time-turner, waiting for the hours, minutes and seconds to rush by her and dump her back in the "past". During third year she had used the time-turner to get to multiple courses so she could expand her knowledge in less time. Five years later, she was using the time-turner for a completely different reason.
As time flashed by her – years turning into a decade – she remembered the number one rule of time-turning: do not, for any reason, go back further than a week. Turning back time in months, or even years, will change history and you will be stuck in that time forever.
After the war nothing was the same. Harry went into depression: he locked himself in his room, refused to eat and never slept. He refused medical treatment and Hermione knew he was dying. Ron was distraught over his brother's passing and wouldn't leave his house or stop prattling on about how great George was – which just made George contemplate suicide that much more.
Headmistress McGonagall was trying to rebuild Hogwarts better than it was before Dumbledore died but it was trying and too many people were slowly losing faith in the government and Hogwarts. The Ministry was doing well, thankfully, but no one knew exactly what could be done about the moral of the wizards and how to boost it.
Hermione promised herself it would get better – and she truly believed it. She didn't tell herself false truths so she could deny the reality of situations; she believed truth is knowledge, and a lie is nothing but the apple in Eden's trees.
Hermione had kissed Ron goodbye, made sure Harry was still in a drugged-sleep from the food she barely got him to eat, and then went back to her muggle home. She dug through her old trunks – the ones from school which seemed like hundreds of years ago – and brought back out the time-turner.
She knew what she had to do: turn back time and reverse the death of Lily and James Potter. She knew it was going to be fine, even if she would never see Ron and Harry again like before; she'd be their parents' age and they'd never remember her as their best friend.
She was okay with all that, just to save everyone. Maybe Harry had to deal with this for more than eighteen years, but it was her turn to shine now. There was no turning back.