Generally every 5 minutes and 15 changes but I do adjust it in some projects.
Autosave won't save while the transport is running, it will save as soon as it stops if one is due.
It also doesn't overwrite the original project or affect it in any way, you can still Ctrl + S as often as you want. It will create a completely independent copy of the project.
Personally, I can't think of a reason not to use it.