Depends on where you are and how unusual your tire size. Sometimes a tire service provider could be minutes away, sometimes several hours.
These providers mostly take care of OTR truckers, will stock the tire types and sizes used on that class of tractors and trailers. What they don't stock, they can usually get in a day or two, unless they are really far from their distributors. You might also be expected to pay the exceptional freight cost (there's a big difference between putting a tire on the next truckload delivery, and getting it by next-day air). This can especially be the case if you are wanting a specific tire brand and model, not just something that fits.
If your tires are atypical, you might be better off carrying a spare one, but that also needs to be worked into your tire replacement plan.
One trip it cost me a half a day to have a set of common size tires replaced on a minivan (three of the five started bubbling off tread, within five hours of each other). More time than that, because it meant stopping early that to be where there was a large tire dealer, and catching him as he opened in the morning to work me in with his scheduled appointments.
Another time, not on a trip, it took three days to get a set of (not unusual, but in short supply) replacement tires for my RV shipped in from the nearest supplier that had them.
So I would expect delays of a day or two, if I need to buy RV tires.
