In many cases, that is true.
If you take a bounty campaign which lasts a long time, you should expect more rewards.
If the rewards remains relatively the same, you should quit.
If the ICO is not likely to succeed, you should quit too.

Well, some bounties have been running for several months, as more and more people have participated in these bounties, so your rewards would also be fewer. It would be better to participate in bounties that don't run for a long time, you would get good rewards by the end. If the project is good then there is no need to run bounty for a longer period of time. Some bounties which have run for a long time have also failed.

In general, I agree that the longer a bounty goes the less you will receive. As a bounty keeps running longer, it gives more people a chance to join which means more people are sharing the prize pool. From the project's Point of view in makes sense to run as long as possible with as many people as possible to spread the word. I am weary of projects that keep extending their campaign for months at a time.

I think this is your problem, not for all of us. basically we should be more aware that all that is related to crypto is always at risk and demands patience. I think you can develop knowledge in trade, investment or mining. so you will not always look forward to the bounty hunters

It all depends on the company.  If you participate in a good bounty, then in the end you will get a good profit.

it is not like that. it is always depends on the rules written in the bounty thread.
there is what you call fixed bounty and bounty on ico raised.
when we say fixed bounty, the total number of tokens allocated for bounty is fixed no matter what is the result of the ico while bounties on ico raised depends on how much the ico earned and there is a certain percentage of tokens allocated for bounty.
better read the bounty rules it is vital.

not only that which can affect it, because large income is also measured by how fast the development of the project until it will make the project is in demand by many investors, and it will automatically pump up the price of the coin

I always calculate, how much am I going to earn compared to people who has been participating since the beginning. If the bounty fund is big enough and project collects enough money, then you might want to join bounty despite being late.

Yes it is,  but is not just the longevity of the bounty that matters but also your ranking decides on how much you will earn after. I guess you should have to stay because atleast if you will stay longer your earnings will add up even if you don't have as much as the highest rank atleast you will be paid even more than quitting the bounty.