Even in the absence of payment, a lawsuit is not an inevitability. Lawsuits are filed when collectors believe they can collect something by obtaining a judgment, not indiscriminately against every debtor. The time lag may be almost nil (from a lawyer's perspective) in the case of an obviously-solvent debtor--perhaps even as early as 120 days of delinquency. In the case of a debtor who does not appear to have the means to pay, the creditor may sell it down the line, and the buyer may evaluate it for suit occasionally and perhaps sell it down the line again without having done anything else. Many debts sell four times. The last buyer is typically a "Buffalo" type collector (who will drive you crazy but will never sue) or a small, local junk debt buyer associated with a local attorney's office (uh oh ... that's the kind that DOES sue).
Some debt collectors never sue. Some don't even contact you, they just buy and sell debt like stock and bond traders. Some report the account to one or more credit reporting agencies (CRAs) and wait for you to contact them about this. Some just sue you and then try to settle or get a judgment and collect.
Each company has its own policies and business model. Sometimes it even varies by the location of the consumer, how much is involved, and whether they think you are able to pay. Each collection agency must comply with the Fair Debt Collection Practices Act (FDCPA) if they regularly collect consumer debts and this debt is primarily for personal, family, or household purposes.
My law office makes sure that debt collectors comply with the FDCPA. If you suspect you have been harmed by illegal collection acts of a debt collector, please contact me as soon as you can. The law permits consumers up to to one year from the harassment to file suit. Documentation of the harassment is very helpful, but sometimes the harassment was by phone and there is no documentation available. Contact my office to find out if you have any concerns.