Had to replace battery in my 2007 Honda Civic and now it won't start. Like the anti-theft is engaged and keeping it from starting.

Disconnect battery positive cable,place key in and turn to the on position,leave it on and replace battery cable,then cut key off then back on.

If key was in igintion when you disconnected battery,you may have damaged the key,if you done this,you may need new key,or hopefully have a spare one.

How long do I leave the key in the ON position before I replace the POSITIVE battery cable?? Is there a certain length of time for it to be able to reset itself??

Ten Minutes,for reconnection of battery.