The average length is 7 hours and 45 minutes, not much different from what most people need.
Literal
Average length [topic-は] 7-hours-45-minutes [with-で], humans' many [subject-が] need time [with-と] greatly change-not.
平均的長さ is 'average length/duration.' 大して変わらない means 'doesn't differ much' — 大して is 'greatly' in negative contexts. 人間の多くが必要とする時間 is a relative clause: 'the time that most humans need.' A factual sentence about sleep duration.