It depends on what you put in them. I hollowed out some wine corks & put cuttings of various plants of micro-stature. Some last for a couple years, some plants outgrow more quickly. They dry quickly & need to be braced if outside so they can't blow away.
plants to consider:
Sedum album, rubrutinctum, burrito
Phedimus, various species
propagated leaf-babies from smaller-stature rosette plants like Echeveria, Sedeveria, Graptoveria