Josh, a client is not going to hire you based on how much you bid, unless they are a lowball client, and you should avoid those.
A client will pick you if they think you are the best value. Sometimes the best value is the highest bid.
The types of jobs I bid on are complex, so what I do might not apply to everyone. But what I do on fixed-price jobs is this: I bid at 100% of the client's proposed budget. And I explain exactly what I will do for that amount, customizing the level of work I do to the budget, while making it clear that there may be additional functionality that can be implemented for additional cost.
@Ahmed S wrote:
during making a bid , the "estimated duration" located above the cover letter ,what does the cell "estimated duration" refers to?
They want you to guess how long the project is going to take. It's a real problem, since often jobs are ongoing based on client needs you can't know about, and often there isn't enough information to determine how long a fixed project will take, and often it isn't clear whether the client is looking for a finite project or continuing work, yet your guess as to how long the work might take if you've guessed correctly about all of those issues and more is part of the information the client receives and uses to evaluate your bid.