Quotas are the invisible force behind many PR timelines, and understanding them explains why a complete, approvable application can still wait years. Systems handle volume differently: some countries set annual caps overall, per category, or per nationality, so excess demand forms a queue that rolls forward; points based systems manage volume through periodic selection rounds where only scores above a cutoff are invited; and some categories, like parent sponsorship in several countries, are so oversubscribed that lotteries or multi year backlogs are the norm.
Realistic expectations therefore run in months for uncapped or priority streams and in years for capped ones, and the honest answer for any specific case is the official processing time page most immigration authorities now publish.
Planning advice: apply as early as eligibility allows, keep your underlying visa valid throughout, remember that rules can change between submission and decision, and if more than one route fits your profile, compare their queues, not just their requirements.