If you don’t bother with a replacement then it will take no time at all.
Depends on what they mean by “replacement”. It’s not like there was nothing before the ACA, after all.
...you'll never be re-elected.
But there will be a replacement per Trump since he said it thousands of times. It will be market based, hopefully with vouchers, but it is not possible to transition overnight. It will take years. After all, Obamacare is not yet fully implemented.