You're not looking for a specific number so much as you're looking for a _steady_ number.
First off, make sure you've given the beer enough time. If it's only been on a few days, forget it and come back later.
If it's been 7-10 days at least (lots of folks around here, myself included, don't touch their fermenters for 3-4 weeks) then you can start checking gravity levels. What you want to see is the SAME gravity number for at least 3 days in a row. Maybe it finishes high (I'm going to be kegging a porter that finished at 1.028 tonight - absurdly high, but I think I know what went wrong), maybe low (another brew's recipe stated I'd finish up around 1.021 or so, but I actually finished at 1.014), but you want to see that it keeps the same gravity for at least three days. Test it one day, test it again 2 days later and, if it reads the same gravity, you're ready to keg/bottle/secondary (if you really want to, anyway - won't get into _that_ debate here!).