Sufficient training for you to be happy to send the piot solo on the type of aeroplane for which the differences training has been given, I would say.
The amount of such training is not specified and will vary hugely. If you fly a PA28 and want PA28R differences training - not much. But if you want a check out on a Rockwell Commander, then probably rather more so.
There is no regulatory requirement for difference training to be monitored by a CFI or HofT; however, it would be more sensible for a FI or FI(R) with a reasonable level of experience on the ac variant for which difference training is being sought actually to conduct it. It will also need to be signed off in the recipient's Personal Flying Logbook.