I was told lots of different timeframes by different people at the hospital. When I had my scan and found out I'd had a MMC, the sonographer said that they usually recommend waiting for your first normal period. The doctor I saw there said to wait about 3 months and the anaesthetist told me 3-4 months (don't know why I asked her as she isn't really a fertility specialist). It didn't matter what they said though as I knew we'd start trying again straight away as I didn't think I'd fall pregnant on the first month anyway. I've had 2 normal periods now and am now in my 2ww to find out if I've been successful this month.
They did give me a leaflet from the Miscarriage Association which said there should be no increased risk of miscarrying again if you get pregnant without even having a period first (unless there is some medical reason for miscarrying), although they did recommend waiting for your first period. But I think they say this because if you did get pregnant without first having a period, then they would have to scan you to find out how far gone you are which they obviously don't like doing. I didn't care though, I pay my taxes so I thought if it happened, it happened.
Sorry about your loss and I hope your next pregnancy is a happy and successful one xxx