Last year I promoted E3T bounty project for 4weeks duration and I earned 30$
In 2018 I promoted Airwallet for more than 14 weeks and I earned over a thousand dollar
Durations doesn't matter, small duration bounties can pay high rewards and long duration bounties can pay worse rewards, it's all vice versa, what matters is the quality of the project you promote
Bounty with a long duration might not interest me to join. There are a number of bounty campaigns that I know to this day are still running even though they have been started since early 2018 and in my opinion bounty hunters are not profitable because at the same time, hunters can do more than 1 campaign