Caesar - if you need to use a loop (as you have shown above) then you can set the step size to be 150 and try something like
for i = 1:150:length(x)
end
This will work assuming that the length(x) is divisible by 150. If not, then you will need to update the code to account for that. Alternatively, you can reshape the array so that you have columns (or rows) of 150 elements each and then determine the mean of each column (or row). Again, if the length(x) is NOT divisible by 150 then you would need to handle the last few elements separately.
x = 1:1:450;
x = reshape(x,150,[]);