The issue with your piss analogy, is that I'd only rate pissing a T1 ability, where-as your space shield is T7. A better analogy would be trying to piss one of those annoying shit-stains off the side of the toilet, say that shit had sat there 'prepping' for a turn or two, you might just need to hold in your piss until it gets to super-pressure levels in order to blast it away. Now that, is a shit analogy of the prep system. 

Character tier levels are inherently linked to the prep system, because higher tier abilities inherently have a prep advantage over lower level characters in most settings. (This is an adjustable rule, but it tends to make sense, and explains why your piss analogy doesn't hold any weight. That piss attack has to face maybe seven passive preps to overcome that shield, which is never going to happen, so we never have to see a logical fallacy that wide. Not to mention, even if it did break through the shield it would do no damage, because logic does still apply when it comes to damage dealt :D)

The reason why preps can't be relative to time is because in 'game mechanic' terms it would be completely impossible to balance and would break logic hard. The prep system is a gaming mechanic, it is not rooted properly in what is wholly logical, it is simply a balancing mechanic that says 'well, we have two opposing magical forces here that are balanced due to the tier, we can't just let whoever has the fancier description win (nor would that be suitable here, as Corban's sheet doesn't even have any hard references) so what do we do? Well, we could flip a coin, or roll a dice? But that leaves it up to luck. Okay, then we'll make it so a person can pre-plan an attack, they can state the intention to launch an attack in a prior post, and then when it's launched we say 'well, he planned to do that', so it's only fair it overcomes someone acting completely on reflex because all things being equal, the thing with prior thought/energy invested in it should be stronger.

There aren't any absolutes of course, T1 has to be flexible to survive the countless contexts it can be used in. However, you can't lose sight of what it is, it's a system outside of the 'IC' so to speak, a way of gauging if something can overcome something else when those factors are almost entirely abstract. There's some things you just can't know, and the T1 prep system plays the part of a dice or coin flip but gives full agency to the players, rather than dumb luck.

How was my crude humour, by-the-by? Can't fault that shit-piss analogy surely? It made me laugh, which may be more of a testament to the fact that I should be sleeping at 3 am, not typing. 

That's not to say you can't use time as the IC capacity by which your character 'preps' but that's a limiting factor you place on yourself, as regardless of what happens 'fluff' wise with your spell, it only gains one prep per turn as part of the balancing mechanic. 

The thing that's difficult to articulate here is how you -have- to divorce the prep system from IC, and do whatever you have to in order to explain the effects of preps through IC action. You display your character carrying out an activity akin to preparing an action, or charging an ability, but out of character you've gained a counter basically, a counter you can exchange for a determined outcome against an opponent's own actions, but which you buy at the price of risking being bopped on the head while you're distracted, and usually alerting your enemy to the fact that you're up to something.