A couple of years ago we did The Beast starting at about lunchtime, following a previous days riding & previous evenings hangover.
We pootled round and to be honest, took ages. Sure enough the cloud came in and it got dark very quickly – we only had small head torches with us & the last few miles were taken very carefully & a bit foolishly.
If you start about 12 and don't hang around, you should be fine but it might start to get pretty dull towards the end, particularly in the singletrack sections. I'd aim to start an hour earlier just to be sure.
I did the MBR about a month ago after not going there for about 18 months & really enjoyed it, but the Tarw (old Red Bull) is good fun too.