As TDA explained it, there is no settlement period
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For example, if someone buys AMBS shares on a Monday, that trade will settle on Thursday (3 business days). If they then sell those AMBS shares on Thursday, the funds generated can be used to repurchase shares immediately, without any additional settlement period.
Conversely, if someone buys AMBS shares on a Monday, and sells on Tuesday or Wednesday, they would be unable to repurchase shares until Thursday, when the shares purchased on Monday are settled.
This is assuming a cash account. Margin accounts would allow repurchase immediately, but interest on the purchase amount would accrue during the settlement period.