Sometimes you go back to an old application and you no longer think it's worth your time, or you no longer have an interest in it. That's when I'd withdraw it.
If it's an older job posting that really piques my interest, I'll only submit a proposal if they've viewed proposals recently - like within a couple of days or so. Some clients list projects and then they never return to those, oh well. Also, I look at their feedback, what their avg pay rate is, if their payment method is verified or not. All things being equal- i really only do that if the job is interesting enough. I've never really paid attention to how many other people submitted proposals.
Just check one thing is jb posting is relevant to your skills and experience, if yes then go ahead.
Don't run for such clients who say I'll hire within next few hours, they are the laziest persons and not even bother to come back to job post after a week.