The intent of the developers is to support SQLite through the year 2050.
At this writing, 2050 is still 34 years in the future. Nobody knows what will happen in that time, and we cannot absolutely promise that SQLite will be viable or useful that far out. But we can promise this: we plan as if we will be supporting SQLite until 2050. That long-term outlook affects our decisions in important ways.
Cross-platform → SQLite runs on any platform with an 8-bit byte, two's complement 32-bit and 64-bit integers, and a C compiler. It is actively tested on all currently popular CPUs and operating systems. The extreme portability of the SQLite code will help it remain viable on future platforms.
Aviation-grade testing → Every machine-code branch instruction is tested in both directions. Multiple times. On multiple platforms and with multiple compilers. This helps make the code robust for future migrations. The intense testing also means that new developers can make experimental enhancements to SQLite and, assuming legacy tests all pass, be reasonably sure that the enhancement does not break legacy.
Extensive, detailed documentation → SQLite has candid, developer-friendly, and open-source documentation. Docs are written by and for programmers. (A few examples:        ) The extensive documentation helps new developers come up to speed on SQLite very quickly.
Heavily commented source code → The SQLite source code is over 35% comment. Not boiler-plate comments, but useful comments that explain the meaning of variables and objects and the intent of methods and procedures. The code is designed to be accessible to new programmers and maintainable over a span of decades.
Disaster planning → Every byte of source-code history for SQLite is cryptographically protected and is automatically replicated to multiple geographically separated servers, in datacenters owned by different companies. Thousands of additional clones exist on private servers around the world. The primary developers of SQLite live in different regions of the world. SQLite can survive a continental catastrophe.
Old school → Nobody is completely immune to trends and fads, but the SQLite developers work hard to avoid being sucked into the latest programming fashion. Our aim is to produce timeless code that will be readable, understandable, and maintainable by programmers who have not yet been born.
In addition to "supporting" SQLite through the year 2050, the developers also promise to keep the SQLite C-language API and on-disk format fully backwards compatible. This means that application written to use SQLite today should be able to link against and use future versions of SQLite released decades in the future.
Our goal is to make the content you store in SQLite today as easily accessible to your grandchildren as it is to you.
Update on 2018-05-39: Our goal of supporting SQLite long-term have apparently come to the notice of the preservationist at the US Library Of Congress who have identified SQLite as a recommended storage format for the preservation of digital content.