There isn't a "one size fits all" answer to your question. It all depends upon the total debt, the age of the debt, the creditor, and your own financial situation.
In some instances (probably yours), the better strategy is to wear out the creditor such that the creditor is convinced that what you have to offer in terms of total amount and number of installments is the absolute best that the creditor can ever expect.
In other instances, the more effective strategy is to settle quickly but this usually requires a lump sum payment or conversely, financial statements showing you really don't have the financial resources to satisfy a potential judgment.
If you can't come up with a lump sum, settlement isn't really an option. Settlement is about motivation & little monthly payments are not going to motivate a creditor. Once a creditor goes to the trouble to sue you, they know that when they get a judgment, they can attach your bank account & garnish your wages & unless you file bankruptcy, there isn't anything you can do to stop them!
Hope this perspective helps!
My two cents: Based on my professional experience, I have seen creditors settle debts for pennies on the dollar AND take installment payments over period of time. For instance, one former client of mine owed $45K to AMEX and settled for 15 cents on dollar, or $6,750, and paid it over 61 days. Settlement occurred, in this case, when the debt was 11 months old. Point here is: there is no general rule--settlement will depend on various factors, age of debt, creditor/collection agency involved, etc.