Calling all armchair generals! Boudica's Last Stand.

[Renatus]

sounds rather like the Mona operation was taking longer than expected, and Paulinus might be 'detained' for most of the year.

I don't think we need read too much into this.  'Detineo' can certainly mean 'hold down, keep back, detain' but it can also mean 'occupy, engage'.  So the passage need mean no more than that, at the time the tribes were discussing their grievances, Suetonius and his army were engaged in operations about as far westward from East Anglia as it was possible to get.

How long would you expect the campaign might have taken, at its shortest? I would think the need to march all the way from (probably) Wroxeter, building roads and provisioning forts along the route, then building a fleet of barges, then the crossing itself and the conquest of a fairly large island, would not be the work of any less than two or three months.

Again, we should not make too much of this.  Road-building would be no more than pioneers going ahead of the main column and clearing obstacles.  Much of the time, advantage could have been taken of established track-ways.  Any forts that might be constructed would not be permanent but merely glorified marching camps, which could be thrown up in a very short time.  So far as the building of barges is concerned, that would depend upon the size of the force attacking the island but, if I am right about the size of Suetonius' army, the combined engineering capacity of three legions could knock up the required number of what were probably simply large punts in pretty short order.  As I have observed before, it would appear from Tacitus' account that the inhabitants of Mona threw everything they had into opposing the Roman landing and, when that failed, they had nothing left.  After that, the conquest of the island was really no more than mopping up.  Of course, all this depends upon when Suetonius started out but, if he started sufficiently early (perhaps to achieve an element of surprise), even two or three months need not take him beyond early or mid-summer.