The hour of the bat, hour of the eel and hour of the ghosts are all sometime in the night and come after each other in the order mentioned previously.
The night crept past on slow black feet. The hour of the bat gave way to the hour of the eel, the hour of the eel to the hour of ghosts.
A Dance with Dragons, The Dragontamer
It is likely that the hour of the bat is either around 22:00 or sometime early morning as that is when the King drank his hippocras though as the meeting of the Small Council after the king's death took place later in the same night it's likely this hour is early on in the night.
Long simmering, the conflict burst into the open on the third day of third moon of 129 AC, when the ailing, bedridden King Viserys I Targaryen closed his eyes for a nap in the Red Keep of King’s Landing, and died without waking. His body was discovered by a serving man at the hour of the bat, when it was the king’s custom to take a cup of hippocras. The servant ran to inform Queen Alicent, whose apartments were on the floor below the king’s.
Ser Criston returned to White Sword Tower and sent his brothers of the Kingsguard to summon the members of the king’s small council. It was the hour of the owl.
Only five of the white cloaks were in King’s Landing at the time of Viserys’s death; Ser Criston himself, Ser Arryk Cargyll, Ser Rickard Thorne, Ser Steffon Darklyn, and Ser Willis Fell. Ser Erryk Cargyll (twin to Ser Arryk) and Ser Lorent Marbrand, with Princess Rhaenyra on Dragonstone, remained unaware and uninvolved as their brothers-in-arms went forth into the night to rouse the members of the small council from their beds.
The Princess and the Queen
From the above information we also see that the hour of the owl is sometime in the night but most likely sometime early morning.
The hour of the ghosts is also said to be in the night and it must still be quite late at night as people have been known to use the cover of darkness for sneaking into places during the hour.
When it came, the fall of Dragonstone took less than an hour. Men traduced by Broome opened a postern gate during the hour of ghosts to allow Ser Marston Waters, Tom Tangletongue, and their men to slip into the castle unobserved.
The hour of the wolf is too in the night at some time, likely around 02:00-04:00 as Cersei uses it to visit Jaime in the night. This makes sense because it is more likely more people are asleep so less chance of them getting caught.
"Cersei." He spoke slowly, like a man waking from a dream, still wondering where he was. "What hour is it?"
"The hour of the wolf." His sister lowered her hood, and made a face. "The drowned wolf, perhaps." She smiled for him, so sweetly. "Do you remember the first time I came to you like this? It was some dismal inn off Weasel Alley, and I put on servant's garb to get past Father's guards."
A Feast for Crows, Jaime I
Though it is likely closer to 04:00 as it isn't long before sunrise.
The day had come. It was the hour of the wolf. Soon enough the sun would rise
A Dance with Dragons, Jon XII
It's also known to be the blackest part of the night which according to @Mooz's answer is just before the dawn.
It was the hour of the wolf. The longest, darkest hour of the night.
The Winds of Winter, Barristan I
Lastly, we know of the hour of the nightingale which is sometime after the hour of the wolf so probably around dawn sometime.
The hour of the owl, the hour of the wolf, the hour of the nightingale, moonrise and moonset, dusk and dawn, they staggered past like drunkards.
A Dance with Dragons, Cersei I
It's worth pointing out that George R. R. Martin has said he doesn't really care about being accurate with things like the size of things so this can be taken into context for times as well. As such it's likely that he's vague with the times and not terribly accurate with them because he doesn't really care about that; he wants you to focus on the story instead.
[How big is Westeros? Is it the size of Europe, or even larger?]
I have deliberately tried to be vague about such things, so I don't have obsessive fans with rulers measuring distances on the map and telling me Ned couldn't get from X to Y in the time I say he did.
However, if you really must know, you can figure out the distances for yourself. The Wall is a hundred leagues long. A league is three miles. Go from there.
But if you turn up any mistakes in travel times by using that measure, let it be your secret.
So Spake Martin, Size of Westeros