There are solid/liquid forms that persist for much much longer. Look at the efforts UK went through to clean up after the chemical attack in Salisbury (which was a liquid in a perfume bottle).
Interesting. I thought the extended cleanup was because the agent was derived from more stable precursors (binary agent), plus the normal overreaction to 'OMFG NERVE GAS'. But the (not conclusive) Wikipedia says the resultant agent is potent for 'years'. I doubt that kind of chemistry stays potent for that long in the wild, but it's by no means my area to judge. I don't think it changes the strategic or tactical view, but yeah...the thought of a year persistent toxic dead zone is unpleasant.
Even the most long-lasting ones will still evaporate in a few weeks. (Of course, if it's inside a bottle it will not evaporate, and if it's inside a city you probably want to decontaminate it rather than wait).