How long a lack of practice does it usually take to forget the intricacies off the learnt Chinese?
As a native English speaker, I've found that after leaving China for four years now, my ability to remember tones has been negatively affected. Second, is remembering words. Weirdly enough, having not read much these past years, I never really lost an reading comprehension (though as before I may forget the meaning of a word and/or its pronunciation). 

The best way to recover (other than returning to China and speaking with Chinese friends- obviously) is to watch/listen to movies and TV as this triggers those neglected neural connections, that you would be surprised to learn are quite resilient.