The reason that the date of separation is important is that everything you acquire after you separate is separate property and all the bills that that you incur is your separate obligation. Sometimes you must continue to reside in the same residence for financial reasons until the house is sold. If that is the case, then in order to create a date of separation, you must cease having intimate relations and generally make a public statement of the end of their marriage by filing for a divorce. Likewise, you may physically separate for long periods of time due to changes in jobs but that is not a date of separation. It is hard to generalize.
Most divorces tend to resolve themselves in six months to a year.
This six month waiting period is an historical hangover from the era when divorces were considered to be immoral. The Legislature put in a six month waiting period to force people to think twice about divorcing. In any event, you can get a divorce now with the marital status to terminate six months after the date of service on your spouse. It is hard to generalize. The key to cost is how many issues you can resolve with your spouse on your own and how many issues require the attention of an attorney or judge.