As part of my S.wales trip I will have 1 day at Brechfa looking at the map of the trails it looks like there is a cross over point between the red and the black which allows you to ride them in one big loop?

Any ideas how long that would take for a reasonable fit rider (I do whites level in around 1.5-2hrs)

Would you bother with the blue / green?

Yeah you can – did it last May I think. Pretty sure we did it in about 4.5-5 hours, but then i wouldnt say I'm all that fit! We started with the Red which was good because you then have the final descent of the red (the best bit of the whole thing!) at the end.
I've also done the blue and green, and they're actually really good fun, lots of potential to get up some good speed. If you have the chance, I'd say do them.
Also definitely worth a visit is Cwm Rhaedr (sp?) which is a 6km trail not far from Brechfa.

Blue/green are largely the same, I would do the blue but not the green as well. It is possible to mix the best of the blue, red and black into one big loop. 4hrs would be a reasonable time.

I'd also say its best to start on the Red, the final descent is the best in Brechfa.

Yeah we usually start with the red – not from the car park where Ian's got his bus. That means you get your coffee/snack break at the bus about two-thirds of the way around (albeit before a nasty climb). We've taken ages over it the last couple of years, but it's been big groups.

The Green/Blue trails are well worth doing IMO. The Blue descent allows you to get some serious speed going and is superb.

Also, we always use the Blue climb when doing the Raven trail as it's more gradual. You just turn left at the top instead of right, and you'll meet up with the Raven trail again, having missed out a very steep, longish climb.

Did this last weekend, started in the Black CP, so finished with a cafe. Have to say it's a excellent loop, all easy climbs (well mostly) and some of the best descents of any of the welsh trails. I think we did it in 3 and a bit hours and we weren't pushing hard.