If your wifi is stable and speed is quite good, then about 2-10 secs for each openrepos repository. If you have any other repositories, it also can take a while for updating.
Major problem may be with nokia OSA repositories, which can lag a lot.
Next warehouse version will update only openrepos (and related) repositories, to avoid such problems with osa repositories.

Although, from warehouse you can fetch information only for specific repository: Goto Settings->Repository-> and Refresh repsository with needed program.

For now you can terminate update with terminal command, and perform manual repository update, to see, where the update process hangs:
execute at terminal, as root:
If it stuck at 'deb-exec /usr/bin/osa' or '/usr/bin/osa --packages'
or other *nokia.com stuff, there is nothing I can do atm.

My N9 hangs on harmattan-dev.nokia.com(which is probably completely dead now) and sometimes on downloads.maemo.nokia.com.
Maybe later it will work better.