If you write a formula for the first 'average' say in column G and row 15, e.g. G15 contains =AVERAGE(A1:A15),
then select cells G1 to G15.
Right-click and copy.
Now select cell G1, then hold down the shift key and select G30000.
All the cells G1 to G30000 are now selected.
Right-click any selected cell and Paste.
This will enter your AVERAGE() formula every 15 lines,
so on row 30 you will have =AVERAGE(A16:A30)
and on row 45 you will have =AVERAGE(A31:A45) and so on.
This is a quick way of repeating your formulas at set intervals.