If it is a make and model that I have worn before - no time at all. I'll might not do speedwork with them the first time or something like that, but I got two new pairs of shoes a couple of days before my last marathon, and ran it in one of them. OTOH, they were a type I have been using for several years - I ran with them once, just to make sure there was nothing wrong, but they did not need any breaking in.
If you are needing to get new shoes, and cannot get the ones you used to get, then get to a running store, talk to them, make sure they let you try out the shoes they recommend, and take it easy for a few runs, just in case.