20 metres approx
Nice one mate, just make sure (I'm paranoid) I can run this straight from my sky dish (which has my sky+ twin cable in) into my freesat enabled tvAs indicated by other replies, Webro WF100 is ideal for the job. I normally buy it from eBay, last time I bought it I think it was about £40 including delivery for a 100m reel.
That will give you 4 cable runs from your quad LNB and a bit left over for some shorter length patch leads if you need any for switches etc.
Personally I would aim for the Webro stuff because some of the other brands use copper foil which is quite thin & brittle, and so cracks off when you try to fold it back over the inner foam insulation before you screw F-connectors over it ( I have some of the poorer stuff, so I "know from experience").WF100 will be fine. PF100 is another alternative, just a different manufacturer.