What usually determines how much medication you need? Body weight? Metabolism? Severity of symptoms?
Probably all of the above. Antidepressants work by initiating and sustaining the growth of new brain cells to replace those killed off
by chronic stress hormone exposure. How much you need is determined by individual biology. Taking too little fails to support this process, however, taking more than needed does not produce a greater response.
How do you know whether you should increase your dosage?
If the dose taken is not easing the anxiety, or depression sufficiently then a higher dose may be needed. You may also need to up the dose if stress levels later increase.
Is it harder to get off the meds and stay off them if you are at a higher dosage?
Not really, though it may taking longer to wean off them. Antidepressants are not addictive, they don't create cravings, so staying off them is not usually a problem. However, both anxiety and depression are often chronic conditions which wax and wane, but don't totally resolve so you may need to take antidepressants again in the future. This is also the case with therapy.
Also be aware that there is growing evidence that antidepressants become progressively less effective every time they are stopped and restarted, requiring higher doses to achieve the previous level of control. They may also produce more severe, and sometimes different, initial side-effects. According to one study
the odds of antidepressants working after each restart drops by nearly 20%. Note that this applies to all antidepressants, not just those taken previously. (interestingly, there is some evidence
that taking a similarly shaped placebo the second time around may produce at least some of the full effects of the original antidepressant)