What is the minimum time for a decent meat marinade? Is there a good minimum time to recommend for the time to marinade meat? Or is it always best to eave overnight if you want good results? 

Do citric juices such as lime or vinegar reduce this time?
It depends on the meat and how it's cut. A 'solid' uncut piece like a pork loin will take longer to marinade properly than if it was cut into thin strips for stir frying, for example - I'd want to marinade the former at least 7-8 hours, while the latter could probably get away with 1.