Don't.
If you start adding Thread.Sleep to "wait" for RX data, then your UI becomes unresponsive, and your users assume your app has crashed. Particularly if it freezes for ten seconds!
Instead, handle the
SerialPort.DataReceived event[
^] and use a Timer to let the user know that nothing is being received.
If you aren't receiving anything, then you need to look at how your port is configured, how the device on the other end is configured, and try to get a response via HyperTerminal or similar first.