Given that the first news of the Bili ape reached the academic community in the nineties and people had no knowledge of this Orangutan population until 1997, I would say that this isn't a long time.
It was known that there were orangutans in Sumatra, but not that this peculiar group in the mountain forest existed.
The Bili ape story is interesting. The link that brought to them being studied were a few skulls collected by Belgian colonists in Congo, preserved in Belgium and classified as gorilla skulls. A photographer noticed that they actually had been found in an area where no gorilla lived, so he set off to look for this new gorilla group. This guy named Ammann got there in 1996, but found no ape, just another skull and a bunch of local legends about very large apes that ate big cats. He also bought a photo by photo trap from poachers.
In 2003 PhD Shelly Williams had the first reported encounter, and filmed some. During the same year, the animals were DNA identified as chimps.
In 2004 a primatologist named Hicks started researching the area. He managed to observe the chimps for 20 hours.
These are very unusual chimps. They have a unique culture, are way larger than normal, and nest on the ground, instead of trees. Their colour also looks really dark to me. For a while, scientists wondered if they were a gorilla-chimp hybrid: they are not.

Also, there might be a new gorilla species or subspecies waiting to be identified in Cameroon, in the Ebo forest. They were first spotted in 2003, and filmed in 2016.