So much would depend on the condition and circumstances, and how much it affects other people.
If you've got extreme social anxiety and constantly decline social invitations because you're afraid someone might speak to you then you can use the excuse for as long as you want, just don't be surprised if people stop inviting you. If you're in a customer facing role and refuse to answer the phone because you're afraid of talking to people don't be surprised if you get fired.
If your conditions have a significant adverse effect on people around you then it ultimately comes down to whether the people around you are willing to deal with it, what treatment might be available and how willing you are to be treated and so on. It's really hard to come up with a simple answer to a question with so many variables.
In a world where just about anything that might be classed as a "disability" triggers all sorts of special treatments it's hardly surprising that some people figured out they can claim a disability and get the special allowances and gain an advantage, when the special allowances were intended to be an accommodation. How to provide suitable support for people with genuine conditions without endlessly catering to the professional whiners who know the right buzzwords to use is the $64,000 question.