well, there are actually 2 answers to this, but surprising that a colleague of sorts is asking this. If you are only making a record change on dns servers it can be like 20 minutes.maybe a few hours. This means that domain .com has its dns on ns1.isp.com and the isp.com admin is changing
www.domain.com from 10.x.x.2 to 192.168.x.2 ( iknow these are all fake ip's or for internal use). BUT if you are moving your domain records to a new dns server such as where domain .com has its dns on ns1.isp.com and is moving it to ns1.isp2.com, that can take a while.still see it take 2 days. As an ISP guy I like to have the dns records for my clients, but I will tell youthe truth, keep your dns records where only you control them. if you don't know dns, then ask and learn. I still do a lot of registry at register.com. expensive, yes, but you can do your own dns there. otherwise i am usin ultra dns a lot lately and have my big multi domain clients there too. that way i am never more than 20 minutes or so from a domain change.