30 seconds gives you time to get blocks, that seems like the most common reason people want an increase in time.
The reason why people don't finish their builds is because they overestimate how much they can build in 5 minutes. In 5 minutes you can build a small house and then add a garden, and it'll look pretty detailed. However if you try to build a big one, like a lot of people do as they don't realise how short 5 minutes really is, they end up unable to finish. It all depends on how big or small you make your build.