Is there any real benefit to a company running an ICO that lasts a long time to attract more investors, or is it best to get it over quickly so that people are encouraged to rush to invest?
If the length of the ICO is stated clearly at the beginning, then personally I don't think that there is any real difference. The issue comes, both for investors and bounty hunters, when the dates get extended/changed during the ICO. This is rarely a good sign, as it most likely means that they are not raising as much funds as they anticipated (whatever the excuse/reason that the ICO manager themselves give). It can be for genuine reasons though, so it does not necessarily mean you have picked a three legged horse.
It is frustrating as an investor when you have invested early because of the bonuses on offer, and then the same (or even better bonuses) are offered during the latter stages. Unfortunately there is nothing you can do, and the bottom line is that you want the project to be successful, otherwise your investment is likely to lose some/all of it's value anyway. I would rather a project extended it's ICO and managed to hit their targets, than finish on time and struggle to deliver on their vision. Some of the best cryptocurrencies started off as slow burners, so a drawn out, support lacking ICO is not necessarily a bad thing, but it does imply that you are probably best waiting until it hits the exchanges as demand is still likely to be low.