That is correct. You have to calculate how much you get from a week's work.
If the bounty duration is very long and the reward is similar to a much shorter one, why would you take the longer one?
Another disadvantage of long bounty is that you risk more time if it is a scam.
It happens mostly but in few projects this is not right because they need more time to launch and complete their product in the market because they are introducing something innovative so the require more time but this happens in very few cases.