I don't think anyone can say for sure when exactly ACARS was switched off unless it sends a "goodbye" transmission but I can't find evidence of that. All we can know for sure is that it was switched off at some point after the last transmission.
http://www.flightglobal.com/news/ar...ommittal-on-mh370-acars-transmissions-396857/
Extract from the above
Presumably the aircraft would have transmitted ACARS data prior to its disappearance, but an industry source familiar with ACARS says this would be entirely dependent on the level of service enshrined in MASs ACARS contract with one of two ACARS service providers, ARINC or SITA.
Use of ACARS among carriers varies widely.
Users of the service can have data transmitted at widely varying intervals, such as every minute to every thirty minutes.
When an aircraft is flying over land, ACARS data is transmitted via VHF to ground stations, but over water the data is transmitted to satellites.
Typically ACARS data has a narrow focus, with engine performance being the most widely monitored aspect of aircraft health. The flight crew can also deactivate ACARS transmissions if they elect to do so.