Cheers chaps

Tomma, clay really depends on several things. Contaminate levels (tar & sap being the worst).

Prep before claying as follows, we always Rinse, foam, rinse, wash, rinse, treat any stubborn muck (tar etc), rinse. Then and only then do you clay. As long as the stubborns have been treated properly, they will come off with the clay, don't rub in the treatment and tar before though If you are using something like Tardis, it's safest to treat and clay it one panel at a time.

I would say to be safe it takes 2 of us claying anywhere 20-30 mins. That is minus doing alloys. We always remove the wheels to clean them thoroughly and get under the arches, brake callipers, suspension etc.