Hi Cesare,

As there is no way to know exactly how long it would take to see the SERP effect on your new change, I actually advice to wait and see for at least one or 2 months. Also, there are many ranking factors, you can't just wait and see after a single improvement (keyword density) to get results. It might be good enough to work; but there might be other factors to consider as well.

My technique is prety simple, after making changes on a particular page, I use a specific amplification for this page (acquisition of backlinks, shares on social networks,...) to force Google Algorithm to not only take in consideration of my previous changes, but also to see this page as a really important one.

One or 2 months might be enough to see what the changes you made means in term of SERP.