Funnily enough I had a teeny amount of spotting at around 7.5 weeks. Mild nausea went at the same time. I'll never know for sure, but I am guessing that it took my body 4 weeks to realise it had all gone wrong ? Am holding on to that thought, otherwise the next wait to 12weeks is going to be even longer ...
As for TTC again - with that mc at 11.5, it took me 5 weeks to have an anovulatory bleed, then I ovulated 3 weeks after that (a week later than my normal cycle. After that, it was back to normal.
Next mc (3 cycles later) was at almost 6 weeks, but the bleeding was very very light - less than normal AF, so I don't think the pregnancy actually progressed very far. That time, I ovulated only around a week late, so fairly normal cycles.
As a rule of thumb, they say that you should get AF within 7 weeks I think, but it can often be sooner - depends on how it has hit you emotionally and physically I think.
this site has some really good info - was my saviour when it first happened to me.
Just remember, your body will be unpredictable, OPK's will give false results in the first cycle post MC and you should continue to do pg tests until you get a BFN - so you know that the next BFP, if it comes quickly, is a new one