Just been reading an old UK T&T publication (by Chris Harvey) cataloguing lots of house rules - very interesting. I got temporarily hooked on his idea of spells taking a certain number of seconds to cast (in line with the WIZ expended). He was using 15 second combat rounds.
Didn't much like the idea of a L3 wizard with a focus casting 15 TTYFs in one round though!
Thought about each spell having its own casting time with reductions for higher level wizards casting lower level spells then remembered a) it's T&T and it's meant to be simple and b0 there's a universal GM fiat.