To take your Breakout idea as an example, the most difficult part in the first instance will be getting the physics of the ball correct. Once you have this and have spent a short while getting to grips with GE you could knock together a very playable breakout clone in a relatively short time, however consider whether you may then want to add any of the following options;
different qualities of brick (i.e. hit once to destroy, hit multiple times to destroy, indestructable)
power-ups (i.e. large bat, multi ball, sticky bat)
extra lives gained at a certain score
hi score table
save game/level unlock functions
in game music
different background graphics for each level
I'm sure you can think of many other things that you could include and each one will take time to implement.
One of the games I'm currently working on, a fairly standard
shoot-em-up is a team effort with others doing the graphics which leaves me free to concentrate on the coding side, this obviously will save me a great deal of time however with a final goal of a commercial release I would be very surprised if we finish in under 6 months.
As Diana Kennedy points out, there is no substitue for hard work, what GE does is give you a great set of tools with which to make sure that your hard work produces good results. I'd certainly recommend going through all the tutorials and having a good look through this forum for any subjects you are interesting in implementing (for instance the ball physics).