I agree with you that good campaigns should be long, also I accept that the payment is less if the campaign is longer, when the campaign is long and there is no minimum number of posts the participants feel comfortable and the quality of the posts is higher because they will not be in a race against time.
But I think that the length of the campaign is not determined by the campaign manager, but by the advertising company. The campaign manager can, of course, request that the campaign be long, but the advertising company does not have to agree to the request because it may have a limited budget for the campaign.