2002 Chevy Impala 0400 code

First off you can not reset the computer by unplugging the battery on a OBD2 system you need to have it cleared. To do that I would call around and find a parts store that does free scans and scan it and when the scanner is done it will ask if you want to clear and clear it. Now as for hte light comming on it may be hours day's weeks or not at all there is no set time usually however it would be within 5 to 10 miles.