When should I apply for Social Security Disability Benefits?

I would encourage to apply when you can no longer sustain employment due to MS and you anticipate that you will not be able to work for the next 12 months. Given that MS tends to wax and wane this may well be a difficult question. There is generally an offset of social security benefits and private insurance. Finally you note your counselor has advised you to apply. I strongly encourage you to discuss this with doctor(s) as I understand you are seeking disabilty based MS as opposed to a psychological diagnosis.

You've gotten some information that's just a bit wrong. Let me clarify. Assuming that you are applying for SSD benefits (the benefits you're entitled to because you have been paying into the system via your employment), your back benefits can go back as far as a year prior to your application date, assuming that you weren't earning income for seventeen months prior to your application date. I know that's very confusing but it's because there's a 5-month period in which no benefits are paid, even though you've been found to have been disabled and legally you're eligible for back benefits for one year.
Now, how does that affect when you apply? Well, as long as you are still earning an income of any significant size, you cannot be found disabled. So if you apply while you're still earning that income, not only will you be denied but even when you are ultimately approved, your benefits will not go back to cover a period when you were working. You may want to consider getting the help of a good local lawyer who prepares initial applications to help you understand the offsets with long term private disability and/or state disability.
Good luck!

I agree with both prior responses, and I would add this: Keep a journal of the days you have had to either miss work or leave early and note significant symptoms or periods of relapses. If you have relapsing-remitting MS (as opposed to progressive or secondary progressive), as months pass it will become more difficult to remember how many days you left early or how many days you suffered severe symptoms and how long the lasted. Establishing the date of onset of disability due to MS can sometimes become an important and/or contentious issue.

It is always advisable to apply sooner, rather than later; however, if you are still able to work, Social Security may agree that you are medically disabled, but may deny your application because you are "engaging in substantial gainful activity", i.e. working. If you are earning more than $1,080 per month, SS will conclude that you are able to work and are not disabled.
Social security (SSDI) benefits are retroactive for 1 year prior to application, but there is a 5 month "waiting period" before you can collect. If you are approved for disability coverage through your private policy, as well as through SSDI, the private policy benefit will be reduced, according to the terms of the policy.
You may want to hire a social security representative to help you sort this out, and figure out when to apply.

SSA wants you to apply when you have been off work 6 months and expect to be off work a minimum of 12 months