F28069U USB: interrupt frequency of IN requests in CDC devices

Hello All,
I searched the internet but didn't get this information:
how often a PC sends out IN requests to a CDC device?

this is how I have understood virtual COM port working. correct if I am wrong:

• since there is no specific request from the user of a virtual COM port, host PC periodically sends IN requests.
• firmware in the CDC device copies data into the USB buffer.
• when the next IN interrupt request occurs, the data in the USB buffer is sent out.

my measurement says 1ms. is this a standard?
Could some one help me here?

thank you,

--
Vishwanatha