As above, doing arches is part of the "detail" in detailing IMO. If you were gonna do the car properly, I'd do it as part of it. For me, I'd only bother if it were pre-show eg night before. Otherwise it's 2-3 times a year max.
Durability is how long the product lasts in terms of time, not how much you use. Or did you have to keep topping it up?