When I had my first mum cat fostered through CP, their advice was to give her two weeks after the kittens were rehomed for her milk to dry up, and then get her spayed. I think a month is too long, as you are risking more than one heat, and every heat does increase the risk of pyometra and cancers. I have had mum cats spayed when they have still had a slight bit of milk when it has been necessary and there haven't been any major issues.