How long can a non citizen live in Spain?

Could you please clarify for me, how long is it permissible for a non-citizen to reside in Spain? I'm particularly interested in understanding the visa requirements and any potential residency limits that may apply. Is there a specific duration that a non-citizen can stay without obtaining a long-term visa or residency permit? Additionally, are there any conditions or exceptions that might affect this duration? I'd appreciate it if you could provide some insight into this matter. Thank you in advance for your assistance.