How long after 90 days in Spain when can I return?

Could you please clarify for me, after spending 90 days in Spain, how long must I wait before I can legally return to the country again? Is there a specific period that applies, or does it depend on various factors such as visa type or purpose of visit? I'm trying to plan my next trip and it's crucial for me to know the exact regulations surrounding this matter. Could you provide me with a detailed answer, taking into account any relevant information that might affect my ability to re-enter Spain?