I'd like to know if the time to restore the database is equal the time using
restore validate database. If not, How can I know how much time the restore will spend?
RESTORE VALIDATE reads the backups, but it skips writing the files to the disk.
If writing files to the disk during restore is the limiting factor, then it will be faster. Otherwise it will be the same.
Depending on the backup technology in use, this may or may not be a valid method for estimating the time of a real restore. For example, when using tape backups, the placement of backups and the number of currently available drives may vary.