I've spec'd a few mining PCs using ATI cards, and according to the MHash-to-return estimator on 50BTC.com, most configurations take about 6 months to recover the investment costs.
The 50BTC calculator isn't very useful for this. For starters, it rounds to bitcents.
Six months sounds like a lot if you go buy today's daily rewards. What exactly are you planning to buy?
Is 6 months a reasonable period before making a profit?
Yes. Standard PC components (in contrast to specialized mining hardware) retain part of their value over time. Let's say you spend 1000 USD on hardware today. Six months from now, if your earned 1000 USD and the hardware is still worth 500 USD, you actually earned 500 USD.
However, keep in mind that the difficulty changes every 2016 blocks (roughly two weeks) and that the BTC/USD exchange rate is quite variable.