Great question, with a changing answer: If you're happy with your current application, just keep using it until the hardware breaks. The operating system will be unsupported eventually, but the app will still be working as it always did. If/when you upgrade to a newer piece of hardware (used is always fine) with the latest version of android, just push your Locus or Osmand database over to the new device and you loose no data in the process. Shorter answer: about 3-4 years depending on the device. Mainstream (high sales!) devices get more updates.