Obviously, moderation is the key.
It's as simple as that! I suppose it all depends on what drug you're talking about, as alot of drugs differ in their addictive qualities.
Me?? I usually like to have at least a 5-10 week gap in-between usage, with frequent +++monthly breaks. You truly do feel a better person when you have a long break from using any drug.
But, yeah... as far as I know with drugs like Heroin it could take between one week to a month of consistant usage (depending on person... but usualy it starts off being a "Psychological Addiction" making the person *Think* they won't be able to function normal if they won't use... or chasing that blissfull high opiates can bring forth.
Alcohol:- From what I've experienced, the first few weeks/month of alcoholism won't leave you with withdrawal (THIS IS NOT A FACT!! ONLY FROM PERSONAL EXPERIENCE!!) but then... it all comes down to your genetic make-up, or susceptibality to acquiring an addiction.
Weed:- Hah!! I bet the majority of Bluelighters have been *Psychologically Dependant* on weed at least for a week or so. But, I'd say smoking for about 2-3 months non-stop, every day could lead to difficulties stopping (Again *NOT A FACT!!* personal speculation.)
Drugs like MDXX, well... I don't think it's really a good idea to be going over-board like using every weekend or so, as thinking of the week of work as being shitty because you're not rolling is not a good thing. You don't want to just experience happiness when you're in that altered, blissed up state of mind. Plus, with MDXX people have a tendency to "lose the magic" after abusing this drug for a little too long.
As said before, for maximum enjoyment of getting fucked up on drugs, I would really keep my usage down to about once a fortnight (heh, preferably longer breaks for recovery of those neurotransmitters) Once a month would be the better option though.
I have friends that use like once or twice a year (I used to be like this.) and I tell you what, every tme they do get fucked up on any drug, it's like a mystical experience to them.
But then... we're all bloody different now aren't we?? So I wouldn't go taking this reply as accurate information, just mere advice from a person who has monitored his friends, and his own usage.
Dunno if that helped, take it easy!