How long could a betta live in a 1G without a filter and heater, and with weekly water changes?

The betta would be better off in a filtered, heated, and proper sized tank that's not cycled instead of a non-filtered, non-heated, and too small tank that also isn't cycled.

If you already have the fish then you don't need to cycle the new tank.

Well my Betta lasted six years in just a 1g tank without a filter or a heater. I did change his water weekly and he had a specialized plant made for Betta's which I am pretty sure helped however he lasted very long without a filter or heater.

Of course it can live in those conditions. In an unfiltered, unheated 1 gallon, it could probably live for a year or 2 with weekly water changes. However, as you said you already knew it's definitely not ideal. I wouldn't suggest anything under 2.5 gallons for a single male Betta. But good luck! It will definitely live, and will be very happy once it's switched into the bigger tank!

it should live fine for a few weeks. But why not just put it in the tank that is cycling? It takes 4-8 weeks to cycle a tank, it would be healthier in a bigger heated tank.

Mine live with just a bubbler in a 1.7 gallon for amost a year!

my friends own at least 50 betas that are in like 3 cups of water they live for years.