Not possible to predict, though you can monitor how many seats in business class remain available on expertflyer and the more open the better the more likely some R space for upgrades may appear in the days before or day of the flight. However, this is why I do not waitlist any more. I look for R space at the time of booking, even if I have to do connections (for example, you might find upgrade space to or from BRU, which is a short and fast train ride from AMS). (Also, you can look for cheap buy up offers all the way to boarding time - this is one reason why business may fill up and upgrades not clear.)