With the black tank it will depend also on how much water you put in before you use the tank (4-5 gal. recommended) and how generous you are with water when flushing #2 down. We don't skimp on water anymore and it really makes getting the tank clean after dumping easy and we don't dry camp so dumping more often doesn't matter. For us (party of two), how long it lasts is less important to us than cleaning it.
The grey and galley tank just get dumped when the black is full. The grey tank (bathroom) is only a problem if we take too many or too long showers. The galley tank always lasts the longest.
