The last android device I bought was a Samsung tablet. It was sort of near the end of its sale life, and I got a good deal. However it received only one update after that - 6 months later. It was basically supported for only 6 months. The final straw was the last update rendered the tablet almost unusable - it added several seconds of latency to almost every operation. I'll never buy a Samsung device again.

Note that the Google branded hardware does seem to be supported for much longer. My old Nexus 7 got updates for many years - but the hardware itself was terrible and I couldn't even charge it after the third replacement's micro usb port failed - another typical Android experience, I'm afraid.