We don’t know. I don't think Apple will drop support as fast as it did for PowerPC though. Times are different.
It's true that 2005 PowerPC Macs were only supported until 2009. That's short, but not really unheard of: an iMac from 2000 would be supported until 2005, for example, and a 2003 iMac would be supported until 2007.
The G4 was also a slow and out-of-date chip by 2009, so it would probably be dropped regardless of the transition. The G5 was still good (and 64-bit!) but it only existed in the Power Mac (since 2003) and the Xserve and the iMac (since 2004). So in absolute numbers, those simply weren't that many computers.
Our Intel Macs, on the other hand, will be fast enough for years to come, equally fast as the Windows computers. And there are millions of them.
Also, note that Apple did drop support for a couple of Macs for Big Sur, but certainly not more than usual. They could have perfectly dropped support for the 2013 MacBook Air, for example, but they didn't. So far they don't seem to rush dropping Intel.
I think most Intel Macs will just see their regular lifespan of 7-8 years of macOS support and 2 years of security updates. Apple might also opt to drop support after 6 years, but promise 4 years of security updates for the very late Intel Macs (2019 and 2020 models). But I definitely expect at the very least five years of full macOS support, so the earliest would be macOS 16 in 2025.
I expect Apple to drop support for Intel Macs and for Rosetta at the same moment, similar as when iOS 11 dropped support for 32-bit iPhones and also for all 32-bit apps. So we won't have a "Snow Leopard" which didn't run on PowerPC Macs but could still run PowerPC software.