I think you have to wait 72 hours before rebidding with the same parameters.
If you change something, for example a different class of car or different hotel geographical area, I'm told you can rebid immediately.
Caution: Before rebidding, check all the parameters. Sometimes a parameter is changed automatically (may result in a hotel booking to a far away location) and you are enticed into rebidding immediately.
Was just doing a little bidding of my own and the site said:
Remember - If your offer is not accepted you will need to change your pick-up or drop-off date, time, or car type to resubmit your request. If you only want to change the price you will need to wait 24 hours before submitting a new rental car request.
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