I noticed, if both cited article and new published article is in RG, RG gives notifications of citations very quick. But could not get idea about Google Citations.
I noticed, it took me around 15-25 days in Google Scholar. Yesterday, I noticed through citation alert email ...my citation increased with big number from different articles which were published in different dates in this year.
One my article published last week already included in RG citations (immediately after I uploaded my article in RG) but lets see how long it will take to receive citation alert message from google.