Macintosh data recovery

...is probably your best bet, although I'm sure there are other products on the market. There is no time limit. The limit is imposed by how long it takes before the old data is actually overwritten with new data, just like PC architecture. Hard drives work the same way regardless of the OS.

I've heard it said that if you have enough money and know the right people (like the CIA), old data can be sometimes be restored even if it has been overwritten. All the ones and all the zeros on an electromagnetic storage medium are not identical. Some of them are a little stronger or weaker than others because the older ones and zeros over which they were written provide a tiny bias which can be measured with the right instruments.