I just got on SSDI this year. The time from application to receiving my first check was 4 months.
Yes, get a lawyer. The amount they get is 25% of your back pay, but it is limited to $6000. Once they decide the date of your disability, then they throw out the next five months to calculate when your benefits begin. You get a check for that period, and SS deducts the lawyer's portion and sends it to him/her. You may also have to pay any other expenses (like a consultation with another doctor or something).
Once you've applied, you'll be sent a package of questionnaires. You fill them out and send them to your lawyer, he/she approves them or suggests changes/corrections and then they're sent to the Division of Disability Services in your state. Then an exam might be scheduled. Mine was at an exam-mill (a place that does only these exams, they bring in doctors, the doctor I saw was actually a pediatrician, she probably does these exams one day a week to supplement her own practice). It was about 30 minutes.
It was all pretty simple and painless. The hardest part was admitting that I should do it.
Hope this helps,
Polly