It's a bit of a piece of string one is that. Generally beer is between 1.035 and 1.060 starting.
I made an imperial porter that started @ ~1.096, but have made beers stating as low as 1.038. Generally speaking your beer will finish @ ~65-80 % of the o.g. depending on quite a few factors e.g. if you make a kit and do it with cane sugar then it will finish dryer and with less body and I have had kits end up at 1.004. Saying that my IP ended up at 1.022!
The real rule of thumb is just wait for the yeasties to finish and have a stable SG for 2-3 days and then keg/bottle.