The Bureau of Justice Assistance (BJA) survey defines regular drug use as taking drugs once a week or more often than once a week. BJA defines regular alcohol use a little differently: it is defined as drinking alcohol at least monthly.
The reason for the descrepency in definitions is that alcohol use is socially accepted, in fact socially expected, within strictly defined (though abused) limits. In contrast, drug use is illegal, not accepted and certainly not expected except in abusive, counter-culture and legally subversive scenarios.
Since alcohol use and drug use are so fundamentally different in Western society, it is really impossible to compare them. Yet if you must try to compare them and select one as more accurate and realistic in the context of "abuse," then erring on the side of caution seems a good line to take. Thus the more accurate and realistic definition might be said to be the more strict (thus presumably more protective) one: "regular use" is use once a week or more than once a week.
Bear in mind, though, that the fundamental social and legal differences between drug and alcohol use make a comparison between the two definitions difficult, if not meaningless.