Possible MP drain over time?

Does anyone know a way that it could be set that a character's MP (or SP or whatever you wish to call it) to drain over time on the map screen.  

For example, Perhaps a spell that would reveal secret doors that the character could activate and de-activate at will, but loses MP for every second (minute, 24 secs, ect) that it is active on the map screen.

I've tried messing around with it, but everytime I try to add any sort of time restraint, I throw myself into a never-ending loop of waiting...

Thanks in advance!

Edit:  Oops!  I forgot to mention that I'm using RMXP  >.<'

using the skill should link to a common event turning skillon switch on

parallel process:

if switch(skillon) in on then
-wait 20frames (1 sec)
change sp(hero who used it or entire party or something) : -1

or something like that

if you do it by hero then make diffrent switches for each hero so that the para will know who to take mana from

Alright thanks, I'll give that a try!  It certainly sounds easy enough.