Don't let people use the expression "Christian liberty" with you when speaking of how to eat, drink, dress, smoke, or whatever. Paul never used the phrase, so you can toss its pseudo-biblical ass out on the street.
I just said "ass".
The concept of Christian liberty is, of course, biblical. But the expression is limited in its use to people who don't like what you're doing, want you to stop doing it in case more people start, but can't bring biblical commands to bear against it. Use of the phrase "Christian liberty" is how they determine that they're within the norm, but you're outside.
I wear my skirt to my ankle; you may exercise your "Christian liberty" by wearing your skirt to the knee or thigh. I'm the normal, you're the weird.
I drink a glass of milk in the evening; you may exercise your "Christian liberty" by tippling over a pint of ale. On the solid rock I stand. You're out there on shifting sand.
I listen exclusively to CCM. I only sing psalms. I'm on a biblical diet. Whatever. I can't prohibit you from behaving the way you do, but I can tell you that you're free to do what's not best by exercising "Christian liberty".
Don't use the phrase to justify yourself. Laugh at people who try to use it on you.
Freedom in Christ is about gratitude and humility. And he has shown you, oh man, how to show gratitude. In several places he has told us to feast, and instructed us to eat much meat and drink as much ale as our hearts desire. We show gratitude by enjoying what God has given us.
Go, eat your food with gladness, and drink your wine with a joyful heart, for God has already approved what you do. Always be clothed in white, and always anoint your head with oil. Enjoy life with your wife, whom you love, all the days of this meaningless life that God has given you under the sun—all your meaningless days. For this is your lot in life and in your toilsome labor under the sun. Whatever your hand finds to do, do it with all your might, for in the realm of the dead, where you are going, there is neither working nor planning nor knowledge nor wisdom. (Ecclesiastes 9:7-10)
To refuse to enjoy the good things that God has put on earth for us, especially those specifically named by him (i.e. wine or beer or meat, but not tobacco), is pride. If you use the expression "Christian liberty" in this context, I charge you to desist.
If you're wondering on what basis I think it wrong for Christians to smoke marijuana, check out this post or this video.