How much time it takes to recover a Firebird database after a power failure?

Short answer: When you use forced writes on your databases (the default setting) there is usually no need for recovery. You simply continue working.

Long:

Firebird is a MGA (multi-generational) database and keeps all the record versions in the database file. There are no log files like with other database systems, and there is no need to roll those logs forward to get the operational database.

If the database file is not damaged (i.e. the filesystem is clean) then your applications can start working at once. The transactions that were commited before the power failure are written to the disk. This is one more reason to keep the transactions short (see the FAQ #42 for more info).