Complex sentence: Researchers estimate that about 70--80% of Yemenis between 16 and 50 years old chew khat, at least on occasion, and it has been estimated that Yemenis spend about 14.6 million person-hours per day chewing khat.
Split into simpler sentences: Researchers estimate that about 70--80% of Yemenis between 16 and 50 years old chew khat, at least on occasion. It has been estimated that Yemenis spend about 14.6 million person-hours per day chewing khat.