long story short, if your electricity is expensive (like Australia) using the most efficient rig (and bang for your buck) it can take 14 months to make your money back. That's assuming the variables that work in your favour don't get worse, which they most defiantly will. If Bitcoin increases substantially in value, the story changes
If your electricity is free, you could potentially make your money back by December.
This is calculated on buying...
4 x Radeon HD 5970 (used) $280ea
A new motherboard 1155 (with 4 pci-e, 16x or 1x it doesn't matter) $60
new celeron cpu ($50)
ram 2gb $15
1200w 80+ rated power supply $250
other odds and ends $100.
Such a system could produce 2800Mh/s, which would translate to about 1.5BTC a day.
Now keep in mind I've used A LOT of rough estimates. I suggest you troll through forums for a few weeks, and run a mining program on what ever PC you have now. It'll give you a taste of what mining is, and you'll then know if you've got the MINING BUG or not