Does this just extend the time of your run so that the average hash rate is paid for?
It does, the ups and downs you see are most likely due to NH disconnecting from CK due to so many reasons including other buyers paying more and there is no hashrate left at the rate set by Willi, the limit you set on NH is the upper limit, you are almost guaranteed to not get higher than that speed, but it doesn't mean you won't get 20% or even 0% of it at times, but what you are essentially buying from NH is "shares", from a technical standpoint, it doesn't matter if you get all your shares in 1 minute or a week, your chances and the cost per chance is going to be the same.
on the other side, economically wise, given that the available hashrate is limited per x time unit, creating massive demand in a short period would cause price increases, this holds true at least theoretically, we have seen times when some buyers would pay 5x whatever everyone else is paying and they would suck the entire hash power, those buyers could have bought the same quantity of shares a lot cheaper had they spread their order, instead of renting 1EH for 1 hour, they might have done 0.1EH for 10 hours, the end result of what they paid in total would have been very likely a lot less.
I think it’s generally better to consolidate all the available mining power into a single, strong push to maximize the chances of solving a block. smaller shares are not frequently enough or rejected.
You will still get the same amount of rejected shares regardless, this exact point has been explained here and elsewhere, the easiest way to think about buying hashrate is to think of it as you are buying bullets from the gun shop, your money is enough for 1000 bullets, your hunting right in front of that shop (so we don't get confused with transport expenses), so it's 3 AM, you are blindfolded right in the middle of the Amazon rainforest, your goal is to shoot a green mosquito that speaks Japanese, it makes exactly zero difference whether you fire those 1000 bullets in one burst or sit there for 1000 days shooting one bullet a day, obviously unless the difficulty changes and you are now required to shot the same mosquito that speaks Japanese but has a unicorn sitting on it's back, in that case, the cost per bullet would drop and so your chances vs how much you pay will equalize again.
