There is no reccomended time frame in joining the bounty campaign
It is base on the project and the team.
How will be the ICO and does the project a legit one?
If you will do a bounty for an ICO it is better to start before the pre-sale like a month before then end the campaign after that pre-sale.
Start again before the main ICO and do the same. this will make the bounty into 2 parts.