I converted from an Oz ATPL to a US ATP-MEL about 6 years ago.
Bearing in mind I met all the experience requirements the process involved:
* Class 1 medical
* Pass the ATP theory exam. I used the Gliem & ASA ATP prep books. Took a week of (very) part time study + a few days concentrated once I was at the flight school I used.
* Sufficient flying to be familiar enough with the system to pass a flight test.
Note: The amount of training
required is zero. I chose to have a few hours to allow for it being a foreign system to me.
* An ATP flight test.
All up it took less than two weeks, including for holiday time & a retest. Dopey me had a brain fade & misunderstood US ATC common practices in the flight test & had to repeat an item.
Even with a test item to repeat I can say I found it all quite easy.
At the time I had a couple of thousand hours & had been teaching Oz CPL, IR, multi & instructor stuff + air taxi etc.
Hope this gives you some comparison