Response time guarantees are hard to fulfill even by the big guys. I remember being in a coloc years ago where a power problem took out a row of racks filled mostly with Sun equipment owned by various customers (my small company included). It also caused some impressive damage to a transformer outside.
Even with a Gold support plan with 2 hour guaranteed on-site response, it took nearly 2 days to get our servers replaced and running, but the bigger guys were back up within few hours of the outage. Sun simply didn't have the staff or immediate spares available to move any faster -- their response was still impressive - they brought in an 18 wheeler full of spares the next day.
They ended up giving us some months of free support or something like that to make up for the delayed response.
It depends on how much they are willing to pay. For most, they pay enough to get a phone call within the hour during normal business hours, an email response within the day, and a 24-hour window if something happens on-site that requires my physical presence instead of remoting in.
I do have a couple of clients that pay quite a bit more for a call-and-fix any time guarantee, but I only offer that level of service to businesses where 1: there is at least one person on their regular staff who is technically competent enough I can walk through more advanced issues and they will understand how to follow instructions, and 2: I'm the one that built out their network, so I know the gritty details.
For the times when the shit hits the fan in two places at once, I have friends who do similar work, and there are a couple of guys I can call to cover if needed. Obviously they get most of the money from these instances, but the customer stays happy, and I get larger monthly residuals than I could otherwise take care of, so everybody wins.
Even with a Gold support plan with 2 hour guaranteed on-site response, it took nearly 2 days to get our servers replaced and running, but the bigger guys were back up within few hours of the outage. Sun simply didn't have the staff or immediate spares available to move any faster -- their response was still impressive - they brought in an 18 wheeler full of spares the next day.
They ended up giving us some months of free support or something like that to make up for the delayed response.