You should not apply for SSDI while working, or while looking for work. They make it very clear that your application won't even be considered if you can work! NO meaningful, gainful employment, even employment for which you might not be trained to do, or for example a low-skilled job when you have advanced training and skill.
If you are considering now that you believe you can't work anymore due to disability, start NOW to be sure to talk with your doctor about it. If he doesn't agree, your application won't go anywhere. Make sure any additional doctors also know, and agree (like neuros, ophthamologists, chiropractors, any other medical people you might see). They'll ALL be asked by the social security department, and they'll all have paperwork to fill out on your behalf.
Make sure that you've done what you can to retain your present job: have you informed your employer of your disability? Have you asked for "reasonable accommodation" under the Americans with Disabilities Act, and has it been granted (or denied), and if so, why or how? Have you thought about other ways you could do your job, or move to another job within the company (even if it pays less, or has fewer benefits)?
The reason that the applications for SSDI take so long are many: 1) they are way understaffed and overworked; 2) many more people apply for benefits who really aren't eligible to get them, so there are lots of applications that are automatically rejected; 3) the social security people are very skeptical of applications, primarily because many more people apply who really aren't eligible; 4) money is tight..everywhere, even in government...and they want to try to make sure that people that REALLY can't work because of disability are getting the benefits; 5) once you do apply, you'll have lots of paperwork to complete, as will your doctor(s), employer, sometimes friends and neighbors, sometimes spouse; there'll be doctor appointments you'll have to go to (at their expense, but your inconvenience), there may be interviews, and second interviews, and so on.
Presumably you know for certain that you're even eligible to apply? That you've worked for more than 10 years in jobs where you've paid into the social security system?
I don't mean that you should provide me answers to these questions...more that these are questions you'll be asked by the SSDI examiners, so it's good to start thinking about them now, and figuring out how you're going to answer, should you decide (or should it become quite necessary) to apply for SSDI.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....