Autosave will only be invoked when LMMS is idle, so it will not by default happen in replay or during export! You control the interval between each Autosave. Because an autosave results in a tiny drop in performance, you find the settings for using Autosave in Settings > Performance. A choice of 5 - 15 mins is recommended. The default is 5 mins. If LMMS crashes it is very important that you pay attention to the screen next time you open LMMS! You will get 2 options from Autosave: