May I suggest, that after dumping and after adding about 2 gal of water to your black tank, and when adding the degrading powder/liquid or packet down the toilet also add 2-3 caps full of Calgon liquid (available at most grocery stores or WalMart). This helps when emptying the tank as the water is soft and it all slides out easily.
We just returned from a month long trip out west and we have a 45-50 gal (I think)black tank and we emptied after each stay of at least 3-5 nights. A lot would depend on how much we used it - if we were out sightseeing all day or spending time around the trailer and using the toilet more. Some places we were stopped for a week others 4 nights.