I did this about 8 years ago but doubt that much has changed...came from a similar position as you're in... The 'written' is a bit of a joke... done on a computer and is little more difficult than you'd expect from a PPL exam in the UK / Europe... however... having said that, the oral you undergo with the FAA examiner who afterwards will fly with you on the flight test can be tough.. he/she will have the results of your written and will know of any weak points... they'll give you a hard time if they suspect you're not up to speed and the more weak areas they discover the harder they'll dig. normally you can expect a couple of hours for the oral but I've heard of them going on for as much as 5 hours when the examiner wasn't happy... and following that grilling you'll have to fly the twin for 2 plus hours... I've heard that this can be easy or tough depending on the examiner... mine was pretty tough... I came from a twin turbine background with no piston experience worth a damn and none in the previous 10 years so playing with stuff like cowl-flaps/mixture etc was all adding to the fun...
One of the strange aspects was not knowing exactly what the examiner would want demonstrated on the flight test... In the US it's unusual to sit the ATPL as we do it... mostly new guys do the CPL then push off to fly night-freight or whatever...or they sit right seat on a feaderliner and upgrade to the ATPL when converting to the left seat... you may find the same problem getting info or maybe I was just unlucky... I went 1st to Comair in Fl and discovered that NOBODY there actually held and ATPL other than the big boss, long retired from the airlines ..and his had expired ! They ran out of suitable aircraft due to maintenance issues so I swapped over to a place in California and more or less made up the training for myself... Good Luck ..