Accurate forecasting eruptive activity is a core challenge in volcanology. Here, we describe an end of eruption forecast during the 2021 La Palma eruption using continuous GNSS geodetic data. We observed that co-eruptive deflation resembled a quasi-exponential trend and used it to forecast when deflation would cease equating to when the eruption would end. The forecast was done within the eruption, however was not operational due to large uncertainty in the unknown pressure dropt threshold needed to stop magma upflow. In hindcast, we explore minimum datasets needed and how forecast uncertainty reduces with increase in ingested data. We conclude that forecasts could be possible after the time-scale around the e-folding time of the exponential decay and quite accurate ones after twice that time-scale. Our results also indicate that the eruption was controlled by the dynamics of a Moho depth reservoir beneath Cumbre Vieja volcano.