Nearly all spells are standard actions (notable exceptions: Enlarge Person and summoning, but there are about two million ways to summon as a standard action so let's just say Enlarge Person).
How would it change the flow of combat if higher level spells took longer to cast? Impressive magics might take a little more concentration. As the martial characters need to stand still starting at lvl6 to take advantage of their new powers, what if we had something like that for casters?
Without putting too much thought into how to divide it, how about (on a full caster scale):
Lvl 1-3 spells: Standard action
Lvl4-6 spells: Full-round action
Lvl 7-9 spells: 1 round action
And this list would not be without exceptions, certainly mages will want to research some stronger spells that can be cast a little more quickly, but they might not be as strong. Or perhaps introduce a "rushed casting" option that reduces the damage/buffs/debuffs/duration time of the spell.
Currently casters get only more mobile as they grow in power. What if they had to stand still like everyone else?
Don't forget the Summoner's SLA.
And hm. I kinda like the idea for some things, but I think it should be based on TYPE of spell rather than LEVEL of spell.
Battlefield control and summoning, a full round action is good.
Healing spells shouldn't take longer than a Standard, ever.
TBH I think a good buff to Evocation would be making more Evocations Swift actions, with a special clause that they can use the Standard to cast another Evocation (and ONLY Evocations).
Evocation was never the type of spell that made casters so powerful, it'd be nice to see something that would allow for some more flashy battle caster stuff. Good enough that it doesn't overshadow martials in every way, but attractive enough that it might tempt some people away from battlefield control and summoning and so forth.
Casting time based on level reminds me of how Palladium Books does their casters. But they don't use the Vancian style of magic so they have a lot more freedom when casting magic (and can learn magic pretty easily, or at least most classes). Spell casters in Pathfinder do use Vancian style meaning they don't have as much freedom, they have to predict what spells will be most useful at the time.
My point being, if you add one system of limitation then you'll have to remove a system of limitation or you remove any reason for someone to play as that class.
It would definitely be better to do this on a case-by-case basis rather a blanket by-level approach. But I'm just suggesting this theoretically here, so I put in some rough boundaries.
While I agree that Evocations aren't the most powerful of caster options, I'm not so sure that they need free quickens to be viable.
My point being, if you add one system of limitation then you'll have to remove a system of limitation or you remove any reason for someone to play as that class.
Besides the fact that it's still magic and still does amazing things?
How about doing the opposite: Turning the full attack into a standard action instead of full round?
It can do the most amazing thing in the world, but if it's not practical to use, why bother?
And no, I don't agree with the opposite either. If a full attack becomes a standard action then you'll just see a bunch of fighters going around slicing things up instead of using tactics.
If you really want to limit casters based on time, then you should adopt the spell point system before expanding the casting time. I'm not sure if there is a Pathfinder book with that system, but it's in Unearth Arcana.