How long does it take to earn back the investment of a miner?
You can run the miner all the time, not just overnight or while you're at work.  It doesn't use hardly any resources on the laptop... all the heavy lifting is done on the miner.

It's very risky to buy a miner.  The miner manufacturers often take a long time (i.e. several months) to deliver your order.  While you are waiting the mining difficulty is going up and the manufacturers move on to new more powerful models.  As a result, by the time you get your hardware it could be well on its way to becoming obsolete.  Keep in mind that a mining rig is not useful for any other purpose.

It is a very real possibility that you will never make back your investment unless the bitcoin price goes up a lot.  And if you want to bet on the bitcoin price going up, you can simply buy bitcoin and save yourself the whole mining hassle.