Volcanic aerosol can persist in the stratosphere for many months and have a significant cooling effect on the troposphere. The effects of the eruption of Mount Pinatubo in 1991 are an important recent example. The time that aerosol remains in the stratosphere is determined by the combined effects of transport and sedimentation. This paper sets out a simple conceptual framework for capturing these effects and combines it with observations and modelling to give robust estimates of key timescales. This framework will be valuable in studies of future eruptions and in studies of artificial stratospheric aerosol injection as a possible approach to solar radiation modification.