It depends on how often the author checks ResearchGate.  Some authors are only here rarely.  

In addition, some authors may not have their article in a form they have permission to share (given the copyright rules of the journal).

Hi Kaylee, I have had my request fulfilled from a range of hours to days, weeks and months. As Michael pointed out, it depends on the author's pace of checking the request.

The best answer was provided in RG by someone (I am unable to recall the name now).

.......immediately to never.