How long does it take for a glycoprotein to be synthesised de novo, processed in the ER/Golgi apparatus and transported to the cell membrane?

 Generally, this process can take anywhere from minutes to hours

Few minutes to several hours.

Best

It's highly dependent on the specific protein, but as a ballpark it is on the order of a few hours. 
There is nothing specific about glycosylation, as compared to other semi-permanent post-translational protein modifications, in terms of timeline for synthesis, modification and transport. 

 The time it takes for a glycoprotein to be synthesized de novo, processed in the ER/Golgi apparatus, and transported to the cell membrane can range from a few minutes to several hours, depending on various factors related to protein size, complexity, and glycosylation.