SEO is much like going to the gym. After adhering to a strict exercise routine for 12 months straight, you're going to look and feel great. Stopping for a month or two might not cause much harm. However, over time you will start losing your great physique and need a tune-up.
Everything we do SEO-wise is permanent and long-term. However, given enough time your competition may improve their efforts and outrank you.
The best thing to do in order to always maintain your rankings is to scale back your SEO expenditures once you reach your goals but never completely stop your SEO efforts.