Depends on the actual GPS hardware but it is reasonable to expect about 10m horizontal accuracy now that SA is turned off (it was 100m when SA was on). Vertical accuracy is usually about 3 times worse than horizontal.
If the GPS hardware can receive WAAS correction it can do better, about 7.6m worst case, 1-2m best case. Most phones don't have WAAS though (do any?).
There is a table of expected accuracy for various kinds of devices in the WAAS entry of Wikipedia.
