I'm not an expert on this stuff but if you're just changing the IPs and not the actual NS records for the domain, the changes will be pretty much instantaneous,

I'm not an expert on this stuff but if you're just changing the IPs and not the actual NS records for the domain, the changes will be pretty much instantaneous, depending on how quickly your DNS service registers updates. I use DNS Made Easy and they usually take about ten or twenty minutes to accept a changed IP; once it goes into their records it's reflected everywhere immediately.