The minimum set of tests needed to provide 100% branch coverage takes about 3 minutes on a fast machine.
To run all of the tests under various compile-time options and on all supported platforms usually takes two to three days, assuming everything works. A full-up automated test on just Linux x64 takes about 18 hours.
The sqllogictest suite (https://www.sqlite.org/sqllogictest) only has to be run on all the various servers when the tests themselves change, in order to validate the tests. Otherwise, those tests only need to be run on the system under test, since the tests themselves contain the correct answer to check against.
The sqllogictest suite takes about 15 minutes to run three times against SQLite (once with all optimizations turned on, and twice more with various optimizations turned off) on a fast machine.
To run all of the tests under various compile-time options and on all supported platforms usually takes two to three days, assuming everything works. A full-up automated test on just Linux x64 takes about 18 hours.