Not often enough.
My ideal would be that one day there would be an announcement from the devs: "Hey people, we made this game. Here's the info, here's the screenshots, and here's the system requirements. Its gone gold. Hope you enjoy it."
Game companies aren't going to do that because they still need to advertise the game, otherwise, no one will buy it, and thus, they won't profit, and as any one of us who knows anything about the world should know, if you spend a large amount of money on developing a game, you want it to turn in some level of profit. Usually, advertising is more likely to be effective if you do it some time before the game is released, because it has more time to build up hype, while hype can sometimes lead to dissappointment on the user's part, it's still an important part of advertising any product.
After Oblivion, I'm actually thrilled that there wasn't an earlier announcement. They released an early build screenshot. They said it was an early build screenshot. They pointed out that nothing is set in stone until the game goes gold. And the OMG THE DEVS LIED threads started, at least one of which was caused by the early build not-set-in-stone screenshot. People were demanding that they see the E3 demo. Even though it was a demo that had the pc ubered with the console, the enemies nerfed with the console, and the ai ramped up to hyper to show off the whole game in 20 minutes, people assumed that the demo was what the game was going to be.
The demo also included some features that were removed in the finished product, note the dynamic object shadows that were shown off in the demo, for example, that's still different from the developers lying, though, because it WAS something that was in the game, just that it was removed before the game was released. Now if Bethesda told us that the game was going to have features which were never part of it at all at any point, THAT might be lying, and even then, that's debatable, because they probably planned to put them in the game, they were just never able to do so, it would just be foolish to say your game was going to have something you'd never intended to include. And if you invited your friend to a party and he said he would come, but then was not able to due to because he had an accident and is now in the hospital, would you say that friend lied? If your answer was no, then it seems just a little hypocritical to me to accuse developers of lying because features they promised simply turned out to not be doable, due to technical, budget or time constraints, or other reasons. It can still be dissappointing when you're led to expect something which the game doesn't deliver, but just because you're dissappointed doesn't mean you were decieved.
I'd say that what Bethesda has done with Skyrim has been pretty clever, though. The way they announced the game gave very little information away, but seems to have been quite effective at getting people excited. We have a trailer that doesn't show any actual gameplay, and has dialog that leaves many things vague, but seems to have caused a fair amount of speculation. It's kind of like how in movie trailers, you often hear bits of dialog that, when taken out of context, don't really tell you what's doing on, but still sound quite dramatic. And since they waited until it was so long into development, they have a lot of fans who are eager to consume any information related to Skyrim, it also means that probably many things, at least in regards to the engine and core gameplay mechanics, are set in stone. So when Bethesda does start revealing information, I suspect that most things we'll hear about are things that aren't going to change too much before release.
In any case, it seems like we'll get a lot of information when the new Game Informer issue is released, not sure what they'll show us, but I'm sure we'll have lots of discussions of it here when the time comes.