I can't say for long term use but I tried messing around with it on two Android devices with Kindle apps and the sync was instantaneous both ways. When I closed out the book on one device (keeping the app open) and went to the app on the other, it would trigger an auto sync, pull down the latest location and there it was. I think I remember it even syncing correctly if I didn't exit the book. The only thing that it didn't seem to do, which surprised me, was auto downloading the latest book I was reading. It appears to keep that part manual. Perhaps there is a setting that changes it but I haven't had a chance to go digging for it. Once the book was downloaded, it synced locations immediately.
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