And just to clarify, this does not mean that you are not able to make projects longer than 42 seconds, it just means that you can only preview 42 seconds at a time.
If you want to see your whole project in realtime, you can always render it out to a file.
And by the way, this 42 seconds is not something fixed, if you use a smaller comp your RAM preview will be longer, and if you use a bigger comp it will be shorter. And the time it takes to render the RAM preview depends on what kind of animation and which effects you use.
To get a longer preview, you can change your resolution to a lower setting.
But I guess this is all in the link that Dave LaRonde provided, so this post is probably quite unnecessary. 🙂