Some people flip right away and make a profit, others flip right away and make nothing. A quality set like the Tower Bridge might appreciate for years. I am not one to recommend quick flips for most LEGO sets. If you have the space and extra money, let the sets sit for a couple of years...maybe even 4-5 on some. You might be surprised the returns you get.
There is a vocal "flipper" coalition on these forums that like to promote that sort of investment strategy. I on the other hand play it much more conservative and look for long term gains. It's very much like stocks. There are many ways to make money, you just have to decided what your risk tolerance and time frame is before choosing a type of LEGO set. My way might not always maximizes profits, but it lets me sleep at night.