This is a very simple script that will send the ctrl+s command every minute while SAI has focus. It only activates on SAI, so it will not ctrl+s while you are doing other things.
Code is taken from here, all I did was change it to work on SAI instead of Spore, and compile it into an exe.
You need to start the exe when you start SAI. (Or you can set up a scheduled event to run it whenever SAI runs, or replace the link to SAI with a batch file that runs both this and SAI—up to you!) It doesn’t have a taskbar presence, so you won’t see anything, though it’ll be in your task manager if you go digging. The first time it ctrl+ses on a file you’ll have to enter a filename. It’s fine to just put in gibberish—you can delete it later! Then click save, and this tool will continue to save as you draw. It will close about a minute after SAI closes, so if you close SAI and start it up again, you need to run the autosaver again too.
It will only save on one-minute intervals when SAI has focus, so in theory if you were excessively multitasking and just never happened to have focus on SAI when it checked, you could go a couple of minutes without saving, plus if you had SAI running in the background for hours, it wouldn’t save in any of that time, but in general this should stop you from losing hours of work from forgetting to save.
If you want to save a backup of something, e.g. before erasing a huge section and you want to make sure you can go back if you don’t like it, please remember to just save as a different filename.