Recently, the German and Italian publishers of XXXHOLiC announced to their readers that XXXHOLiC would end in the current volume (19).
With Tsubasa, the climax phase was first announced in January but the series only ended in September. Personally I think Tsubasa was an unusual case... usually when a series enters its climax, there are only a few chapters left.
We will know soon enough if XXXHOLiC really ends in volume 19. The next CLAMP-NET.COM updates will announce the last chapter of the current volume and we will know if it's indeed the final chapter of the series.
Volume 19 will be released in March 2011 along with a new OAD.