Also, how much ammunition you expend in defense is generally determined by how intense the attacks are. I don't believe Ukraine has the offensive combat power at Kherson that the Russians did at Mariupol, so it is going to take longer for the Russians to exhaust their munitions.
I'm not saying the Ukrainians won't retake Kherson, because I believe they will eventually. I'm just saying that it isn't a magic bullet that can necessarily force a quick victory. A lot will depend on how determined the Russians will be to hold it despite the unfavorable geographic and logistic situation.
I think the Ukrainians are counting on the airdrops or ferry efforts...to take out more cargo planes, and ferries.
The way the Russian army conducts operations they can’t build a latrine without expending massive amounts of munitions removing brush and three small trees. The smart thing to do would be for the Russians to pull back across the river, but the politics won’t allow it. They will stay there and lose men and equipment until there is no escape. The real strategic battle is west and south of Doneskt where these troops could be doing something useful.