Change of Providers - time to migrate IP addresses

Is there a generally-accepted "best practice" that dictates the time frame for relinquishing address space when changing providers? I have a client with a /24 from provider X; we've built the infrastructure for connectivity to provider Y (with new address space from them) but still have a few hosts that we've not migrated to the new address space. We'll certainly be rid of the old addresses within the next 60 days but would like to terminate the circuit to provider X sooner than that. I think this is a pretty typical thing but Googling hasn't turned-up too much in the way of evidence of this. Is this a reasonable thing to do? I appreciate any feedback from NANOGers. Thanks, Ben