One of the highest scoring levels that can be done in 1:30-1:40 is "Inside of Terror", toward the end of the 3rd world. When I had everything 5 starred I had 250 million, and grinding out the other 250 million the way I described took about 30-40 hours with earning an average of 520K per run. Bear in mind that 2-6 doesn't appear to be able to be 5-starred.