What will happen if you apply for a visa, either through a lawyer or yourself, is that the US Embassy will decide whether your crime is moral turpitude or not. Assault isn't always - depends on intent, use of a weapon, whether that weapon is 'deadly' or not.
If it's not moral turpitude, you will be approved a visa at your interview on the spot and sent it in about a week
If they can't decide, they will refer it to Washington - that could take months.
If it is moral turpitude they will almost certainly refuse you a visa. However, they may suggest you let them apply for a 'waiver of ineligibility' for you - again that will get referred to Washington and can take months, though in simple cases it can be less.
Somewhere on the Embassy website will be advice on how long to allow for 'additional administrative processing', which is another way of saying 'getting a waiver of ineligibility approved'. You might also have to allow longer to wait for an appointment for an interview as only certain staff are trained up in these procedure.
The length of your sentence is not really relevant to your chances of a visa.