If you're Instrument current and presently Aircraft Commander qualified in a US military airplane, you can almost certainly handle an ATP checkride in a light piston twin, after a few training rides.

The modern problem is the CTP, that allows you to take the ATP knowledge test. I've no idea how long that course takes. Ask the training provider; I'm sure they have a set schedule/curriculum.

But the ride itself--you can be trained and ready for that in about three days, using a simple aircraft. Try 1-800-ALL-ATPS. They have many locations in the CONUS.