Let’s say the In Time cells are in Column A, and Out Time cells are in Column B, and you want Time Spent to be in Column C. Put this formula in cell C2 (assuming A1, B1, and C1 contain headers, not data):
=ARRAYFORMULA(B2:B - A2:A)
The ARRAYFORMULA function instructs the spreadsheet to iterate the contained formula over the ranges given, and a reference without a final number like B2:B refers to a range that contains all the remaining rows in the spreadsheet.