I want that the PInfo[playerid][JailTime] would be lower by 1 each second, can you help me?
Sure. Just use a timer. Read this page on the wiki. Each 1,000 milliseconds, subtract 1 from the variable via either 'pInfo[playerid][JailTime]--' or pInfo[playerid][JailTime] = (pInfo[playerid][JailTime] - 1). Then every 30 seconds or some other reasonable interval, update the SQL database.