I think you will notice little or no change with a 15-day course.
He had noticed a change in the air of the city.
Teachers noticed a change the first day he took over.
Whether you are going to notice a 1% change in a 50-100 year period is another matter.
He was rather more than a year old when I noticed the change.
When the reading was over, I noticed a complete change in the mother.
But, even as last year, she noticed a change in me.
It has been about 5 months now and I notice a change.
In the office, you don't notice the change of season.
Something pulled me away, although I didn't notice the change very quickly.