Not that long. Everything now is on borrowed time...
My guess would be 2025 at the latest.
Almost all Amiga's I've seen dead were not chip, but board death due to batteries and caps.
However, for C64- it's always chip death. PLA, VIC, ROM, RAM... It all just dies one day. The VIC-II in my C64 just died again (go through about one per year, but it gets daily use and it's on a good monitored power supply).