Prophetic narrations state that Hell will almost be empty and anyone with more faith than the weight of a mustard seed will be removed from it. However, those that do not have even this amount of faith will remain inside eternally.
It was narrated from Abu Sa’eed al-Khudri (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “When the people of Paradise enter Paradise and the people of Hell enter Hell, Allaah will say: ‘Whoever had a mustard-seed’s weight of faith in his heart, bring him out.’ So they will bring him out, and they will come out like burned skeletons, and they will turn into charcoal. They will be thrown into the River of Life, then they will grow like seeds growing in the silt left by a flood.” The Prophet (peace and blessings of Allaah be upon him) said: “Do you not see how they emerge yellow and curved?” Narrated by al-Bukhaari (6192) and Muslim (184).
However, as I said, people who have not even the slightest of faith or have abandoned the concept of monotheism will not receive this freedom, and Hell will be present for them forever.
Indeed, those who disbelieve and commit wrong [or injustice] - never will Allah forgive them, nor will He guide them to a path. Except the path of Hell; they will abide therein forever. And that, for Allah, is [always] easy. (Quran 4:168-169)
This hadith is also relevant but a bit lengthy:
http://quranx.com/hadith/bukhari/Book-97/Hadith-135/
Some extra links if you need more information:
https://islamqa.info/en/21672,
https://islamqa.info/en/96531
Peace.