In L5R 2nd ed, if a shugenja has successfully cast a spell with a casting time greater than one turn, does she have to roll some kind of "concentration check" if she is hit during a casting turn?
In 4th Edition, under Spell Casting Rolls & Target Numbers, there is text devoted to interruptions.
4th Edition also allows you to reduce the casting time by Raising once per Complex Action of reduction (down to one). A Mastery Level 3 spell can be cast in 1 Complex Action by Raising twice.
Other editions may have similar rules. If not, you can probably backport the rules above into your game.