This suggests a slow memory leak. What type of memory threshold was exceeded? Heap or perm gen?
If the threshold was perm gen, then it is possible that the perm gen allocation should simply be increased -- and that it takes 25-30 days simply to make use of enough different features/capabilities to load that many classes. It may, however, be a slow leak, but this sort of issue is very hard to track down. One note in this regard, if using CMS garbage collection, be sure to use the necessary arguments to force this to include perm gen collection (which is unfortunately not the default with CMS garbage collection).
If this is not a perm gen issue, however, then it is most likely a slow memory leak. The only way to get a good idea as to the cause is to grab a heap dump and then analyze this -- or, more likely, have PTC analyze this.