Valve does have a history of long development periods. Other people have already mentioned this, but Half life 2 took 6 years to complete, and it was a great game. Team Fortress 2 took 8-ish years to complete, and it turned out to be an excellent game as well. (By great, I mean polish, gameplay, and general consensus, personal preference aside) They'e on track for releasing Half Life 3, if their past record is anything to go by. They don't call it Valve time for nothing.
I also wouldn't be surprised if they released it along with a new Source engine. Even if they said they wouldn't release a new one for years, that may be when they release HL3. After all, they've released a new Source engine for each major Half Life release. Admittedly, Half Life and Gold Source don't really count as much, but that may be what they're aiming for.
I also wouldn't be surprised if they released it along with a new Source engine. Even if they said they wouldn't release a new one for years, that may be when they release HL3. After all, they've released a new Source engine for each major Half Life release. Admittedly, Half Life and Gold Source don't really count as much, but that may be what they're aiming for.