Honestly, this being RM2, it all depends on the initiative and activity system/option the GM is using. People quoted how it's in the core. Other systems of note IMO are the "3.5. RMCI initiative system revisited" (RoCoIV, p11) where an instant spell is considered 0% activity, 0 IP but with a 50 IP between casting (I personally use this one) and the "3.10. An activity point based turn sequence" (RoCoIV, p15) where an instant spell costs 1 AP, but with a recovery time of (20 + lvl/2) AP. In both cases, one may attack after having cast an instant spell.
...then, you have the case of the RoCoVI "3.1. Second-by-second tactical combat" where the notion of round even disappears.