Since I am one, my answer is obviously: when you are ready to file the claim. Most reputable Social Security Lawyers can do things that the average solid-citizen cannot. However, there are a couple of things to consider. How old are you? The older you are, the more likely it is that your claim will be approved. The rules change as people age because Social Security recognizes that "advanced age" (meaning someone 50+) is a significant barrier to finding new employement. It is no guarantee, but it is a factor that weighs in favor of filing on your own. What condition do you have. There are some conditions that should be approved more quickly than others called "Compassionate Allowances"
In my mind, the biggest factor is how tired, frustrated and in pain you are. A good Social Security Attorney will take the burden of jumping through the Social Security Hoops off your shoulders. Weigh the cost of paying a lawyer against the psychological toll this will take and then decide. I will say, however, that it is always best to get your claim started correctly at the outset. I find it much easier to work with a case from beginning to end rather than try to fix mistakes made by someone else.
There are many factors other than how long you can sit, stand, walk, lift and carry that prevent people from being able to work. "Vocational Experts" testify about these. Some people have to be very close to a bathroom all the time and need unscheduled breaks. Some people have problems seeing anything smaller than a 50 cent piece, but their vision is other wise Ok to drive etc. Some people have terrible skin conditions on their hands that make it very painful to use their hands on a regular basis. Some people are required by their doctor to raise their feet to waist level. Some need to take rest breaks while laying down. A vocational expert will tell the judge that no employer will hire someone with any of those issues. Therefore, even if they can stand for two hours, they still are not employable. Your doctor needs to explain this to the Judge.