I know when we did major updates we actually gave MONTHS in advance if possible.
We actually had S.M.A.R.T. drive errors the other day on one of our machines - and while we make backups, we don't want to just move a site overnight and expect that they can update their DNS quickly etc (some people don't use our DNS) - so we notify customers that there's potential hardware issues that we want to move them to a new machine etc - give them up to 7 days for the move. They get a special link to schedule a move with our admins, and if they haven't submitted the reqeust within the allotted time, then we move them manually and resend notifications etc.
There's more to an upgrade of software or a site move than what can be expected some times. Whenever possible TIME is what people need, and notifying people on a FRIDAY and dong the things SUNDAY does not count as 48 hours - that's the weekend, most people would have missed the notice.
Even when we do cPael updates or Apache updates which could result in up to 5 minutes of downtime while a system reboots, we give a MINIMUM 48 hours notice just so people know we may be offline for 5 minutes.
Communication is key - if you're host isn't doing that, time to move to a new host!