It seems to take forever before my phone become aware that I have read all my gmail messages on my PC.
You can configure the frequency by:
- Going into Gmail;
- Use the
...to bring up the application bar, and choose "settings"
- Press synchronisation settings
- Choose a new value for the "Download new content" drop down.
Personally, I have it set to "as items arrive" and it is faster than the Gmail notifier application on my PC.