How high is the risk that a patent gets challenged after it is granted?

A lot of the risk depends on the leg work you are doing or willing to do. I have been an SME on a few cases that went in front of the patent office and it all came down to the level of detail provided as to why the prior art was not sufficient to have the patent invalidated. In one case, the decisive factor was the fact that it it possible to run a few thousand connections on-die whereas doing the same on the board level would be not only cost prohibitive but also virtually impossible even at the present time, not to mention 15 years ago when that patent was filed. So, the devil is in the detail but the details are what makes or breaks a patent.