I doubt FM will say how long 13 will be available. In the past they general end access to old versions as soon as a new version is released. 13 is still available as of today because I downloaded and installed it on a device. I would suggest downloading to iTunes and or backup your device to iTunes. This way you will have a copy to restore back to your device. Since this is a little bit of a change from the past they may issue a statement of an end date, but I would not trust that they will. My main point here is to make a backup, just incase.
FileMaker Go 13 will be removed from the App Store on 9-1-2015.