The forward facing harnessed seat (we call this "stage two"), lasts between three and five years, depending on your child.
When is it time to move from stage one to stage two? This will vary from child to child, but ideally your child will be OVER two years old AND thirty pounds before you consider moving to stage two. Rear facing is MUCH safer than forward facing, and you do not want to move through the stages any more quickly than you absolutely must.
There are many options for forward facing seats now, and which one you choose will depend on how long you wish to stay in stage two.
Currently the seat that will harness the average child the longest (without going to very expensive "special needs" seats) is the Britax Regent. The Regent is one of MANY seats that have a maximum weight limit above forty pounds. On average, the Regent will fit children up to around seven to ten years old, depending on their growth rates. Smaller children will of course fit in the seat even longer.
Not everyone wants to harness their children "forever", and that's okay. The MINIMUM for leaving stage two is four years AND forty pounds. Ideally, you want to leave your child harnessed until closer to age 6. Why?
The reason we don't want children moved to stage three (the belt-positioning booster) prematurely is because a properly-used harnessed seat will prevent your child from moving out of position. A booster seat will not restrain your child - only lift the child up so the adult seatbelt fits properly. Younger children simply do not have the ability to resist "temptation". The brain synapses that govern impulse control do not even begin to develop until age five. There is a great deal of evidence showing that even at age ten, some children haven't completely matured.
For small or young children, the booster simply does not provide sufficient protection. Children under six are more likely to move out of position, and in some cases even when properly positioned suffer serious injuries because they are too small for the booster to effectively position the vehicle seatbelt.
A very interesting experiment was conducted recently in which a group of children were tested using the same psycological tests performed on a similar group of children in the late 1940s. The findings were surprising. Children now have lower self-regulation and critical thinking skills than children just a few short decades ago. In fact, they're scoring on average two years younger than children given these tests in the initial study. Five year olds have the impulse control of our grandparents at three. Seven year olds are more like a 1945-era five year old and so on.
http://www.npr.org/templates/story/story.php?storyId=19212514
So what does this mean for child restraint use? It means that a five-point harness is absolutely vital until your child is capable of not only understanding WHY sitting perfectly in a booster is important, but that same child must be able to resist the temptation to lean over to pick up a dropped toy, or speak to another passenger or even fall asleep.
Don't rush stage two! Keep kids harnessed until they are at least five, and if you can make it to the 6th birthday, so much the better.