The length of time you and your spouse are separated can play a part in your divorce proceedings under some circumstances, but in other cases, it doesn't matter at all. It depends entirely on where you live, and if you want to – or even can – file for divorce on fault grounds.
Separation as Grounds
In some states, such as Maryland, separation is the only no-fault grounds available if you want to file for divorce without accusing your spouse of any wrongdoing. In Maryland, you must meet the separation requirement, which specifies the time you spend living apart from your spouse, before you may file for divorce. This isn't true everywhere, however. Illinois requires a separation period for its no-fault grounds, but you can live separately for the requisite period of time after you file. Your divorce will become final after you've passed the separation milestone, which is usually six months if you and your spouse are in agreement to end your marriage. Fault grounds, such as adultery or cruelty, typically don't require any period of separation.
Qualifying for Residency
Some states, such as California, allow you to file for judicial separation – commonly called legal separation – even if you don't meet the residency requirement to file for divorce. In these jurisdictions, after you've lived in the state long enough, you can simply amend or convert your separation decree to a divorce decree if you like, so that your separation counts toward your divorce in this respect, as well.