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

In Australia, the main difficulty you will likely face if you decide to file a patent application for your invention would be whether the Patent Office considers that your invention constitutes patentable subject matter during examination. As it currently stands, a "computer technology" invention is patentable if it can be demonstrated that a computer is “inextricably linked with the invention itself”.

A patent is not revoked - one or more claims can be invalidated, whereby any remaining claims remain in force. If all claims are invalidated, then you may have various options to effectively re-validate the patent claims through various post-grant patent procedures. You should consult a patent professional who specializes in post-grant procedures.

In Europe, revocation of the patent is a possible outcome. The follwing statistics before the European Patent Office, may be of interest, suggesting about 1% of granted patents are revoked in post grant opposition proceedings (31% revocation rate of 4.7% opposition rate).

http://www.epo.org/about-us/annual-reports-statistics/annual-report/2014/statistics/searches.html

It depends on the technology and if you have competitors. Certainly business method patents are post grant challenged more than electrical patents. When my clients have a chance of litigation or post grant challenges, I focus on obtaining a robust patent for them during Examination. Please note: this isn't legal advice.

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.