I've known a few people who took DXM near daily for several months because it made them feel more 'normal' and 'at ease in their own skin', but all three of them experienced significant hardship when they finally stopped. They are all able to recognize now that the DXM wrecked havoc on their ability to think clearly and quickly for quite a while afterwards. I wish I could tell you their dosages but I have no clue really, just that they would get several bottles a day and dose impulsively.
Seriously, for your own sake, take a step back and look at what you're asking. You've been on DXM for three days in a row and have now convinced yourself that if you don't take DXM forever you're going to be horribly depressed and suicidal. Maybe you are depressed but DXM will only exacerbate that in the long run.
And the whole "I've been on it for three days and haven't noticed any drawbacks" line is shockingly short-sighted. People on meth can stay up for days and days and feel great, but its when they have to come down that the heavy problems start.
If you are doing this for self medication as you say you *need* to realize that your attitude towards your depression is as much a problem towards overcoming it at the depression itself. You're really setting yourself up for a shit-show a few months down the line that could take you years to recover from.
And I know you may write this off since I can't give you articles and citations, but I have seen a few good friends get much stupider from heavy DXM use, and I know how slippery of a slope self-medicating with an attitude like yours can be.
Please be careful.