What is a good approximation for how long humankind has been on Earth (as homo sapiens), according to Hinduism/Vedic scripture? I have read some texts on how we came to be, but have not encountered any clues as to how long we have been here, in this current form.
A part of the universe consisting of 3 lokas (which includes earth, sun, solar system) gets destroyed and created every 4.32 billion years in a cyclical fashion for all eternity (past and future).
Brahma creates Manu (through mental energy or manasa srishti), and from Manu comes Manushya or man. In this 4.32 billion years, which equals one day of Brahma, 14 Manus come and go, each ruling for 71 cyclical ChaturYugas, where each ChaturYuga is 4.32 million years, which again is split into 4 Yugas of decreasing duration in quantity of 4 (Satya),3(Treta),2(Dvapara),1(Kali), where 1 unit kali yuga = 4,32,000 years.
We are currently 5000 years into Kali Yuga of the 28th ChaturYuga of day 1 of a 50 year old Brahma.
The lifespan and height and strength of humans gets reduced along with the progression of the 4 Yugas. In Treta Yuga, humans were generally ~8 ft tall. In Kali Yuga, they are generally 6ft tall. At the end of Kali, they will be like dwarves. And when Satya Yuga starts again 400K years from now, they will be modern-day giants again.
To answer your question rhetorically, there have always been, and always will be, humans on earth.
In their current form (say average height), about 5000 years.