The amount of time you are using bluetooth doesn't really matter. If you are using it at all, there theoretically is an attack vector.

I say theoretical, because ledger's nano is build the way that even if it is communicating with a compromised device (which also includes a compromised connection such as a hijacked bluetooth connection), it still can not be exploited.
If you double check the amount and recipient address, you should be fine regardless of whether bluetooth or cable is being used to communicate.