Allah describes Himself in the Qur'an as the Compassionate and Merciful  He also reassures us that He stretches out His hands to receive those who return to Him in repentance, no matter how enormous and how ugly the sins they committed are.  Allah says:

"Verily, I forgive all sins unto any who repents and attains to faith and does righteous deeds, and thereafter keeps to the right path." (Qur'an: 20:82).

Since we have no guarantee when our appointed time with the angel of death will arrive, we are ordered to hasten to repent

"Hence, turn towards your Lord [alone] and surrender yourselves unto Him before the suffering [of death and resurrection] comes upon you, for then you will not be succoured." (Qur'an: 39: 54).

Therefore, one must never put off repentance.

Having said this, I should also point out that the most preferred time to ask forgiveness of Allah, according to the Prophetic tradition, is after the midnight or after two thirds of the night have passed. The Prophet (peace be upon him) said, "When the middle of the night or two thirds of the night passes, God descends to the lowest heavens and announces: "Is there anyone to ask of Me so that I can grant his request? Is there is any supplicant to beg My favor so that I can answer him? Is there anyone to ask forgiveness of Me, so that I can grant forgiveness?"  (Reported by Muslim on the authority of Abu Hurayrah).