I agree that with a cartridge type oil filter the sludge trap will need cleaning out a lot less often. However, the type of deposits I've found in them when I've cleaned them out has been very fine, compacted matter. In my opinion, this will build up over a long period, how long is anyone's guess, and will need cleaning eventually.
However, I last cleaned my A65 sludge trap out about 10 years and 11,000 miles ago. It has had a cartridge type filter all this time and I don't intend to split the cases to clean it out again anytime soon. I will probably do it as it approaches 20,000 miles since the last time. I will note how much clag comes out and let everyone know, but that's likely to be in 3-4 years time so watch this space.
In the mean-time ask yourself how long you want to keep the bike and more importantly how many miles you expect to do on it in that period. After a clean out I wouldn't worry for 15-20,000 miles with a filter and that would be 7-10 years at 2000 miles a year!!
Hope this helps put it in perspective.
Chris