IO540 -
For Spain and Portugal it seems to take until just before you leave to arrive (even if you applied 2 months in advance) and lasts for as long as a piece of string. I've had permissions for the remaining period of the permit from the CI, though the last one was for just a month. I suspect that if you ask for a month you'll get it. Be careful about stating the intended dates of your trip, as they may just issue it for that period, leaving you stuck up the .... if you're delayed.
And, has anyone mentioned the 80 Euros that the Belgians want deposited with them before they look at your application?
Hopefully, a move to EASA permits will remove the silly restrictions, but I'm not holding my breath.