how do you tell when it is 'too late' for rooing/shearing?
Shearing has to be done, so never 'too late' as such. What I meant was from the point of view of the hand spinner, a machine shorn fleece, if left late in the summer, will have a lot of new growth. Machine shears cut close to the skin, so will include new wool with the old, last year's, which is what we're after. If you have fibres with both last season's and this season's growth, then when you prepare them for spinning they will come apart at the join between the two. This leaves you with short lengths of new wool tangled in the fleece, called noils, which cause lots of little bobbles in your spun yarn, as they are difficult to remove.
For roo'ing, as Thyme says, it's too late when the sheep has rubbed all the fleece off on fences and gates, or draggled it all across the pasture
For showing fleeces too, the judge will see this feature as being a 'break in the wool' fibres, and it will be marked (far) down.
Cross posted with Bogtrotter