Decided to actually time myself, using the platform clock, and allowed myself 30 seconds per sketch, and no more.
Blimey. Talk about stress. It really focuses you. No sooner have you started than the numbers are clanking round like the bedside clock on 'Groundhog day'. Forget detail. The best ones I managed were of a passenger who had his back conveniently to me. Otherwise I doubt I would have got anything even anywhere near enough to register at all.
Look, I didn't even get time to give him both legs in that one. I would have said I sketch fast, but this proved how fickle my judgement of time really is.