It all goes back to the participants, I think if the project is good then the long duration of the campaign is not a problem for them but if the duration is long and the pay is not appropriate or the allocation is suddenly cut I feel they are very disappointed. So I see very few campaigns now have a duration of up to 5-6 months.
Just hope if the project can be trusted by the participants. I have experienced to see that the participants are getting deceived by the project even if that was getting a big success too. The duration is not a problem at all as long as the developer has a very good track history.