Colibri T20 Hang issue on comm port reading

I am using Toradex T20 module with Wince7 OS version 2.0, I observed OS hanging issue while reading Comm Port. My question is that how many maximum bytes can be read from comm port in a single “ReadFile” function call?

I am reading the comm PORT 10 minutes once and processing the data as per my application. I am suspecting this latency of “ReadFile” function call is the cause for OS hang. I thought OS is unable to handle the bulk unread data that accumulated at comm port. Please suggest a method to avoid hang issues.