Subsequent castings of stoneja do not increase the time period (i.e., if Blm1 casts it and starts the timer and Blm2 casts it again 30 seconds afterward, the effect still wears off 60 seconds after Blm1's casting)
The exception to this is if Blm2 is wearing the Goetia Chausses +2, which will increase the time to 90 seconds after Blm1's initial casting.
Each successive casting of stoneja (even from other blms) increase the damage by an additional 5% damage bonus. (ie: 5%>10%>etc)
It is unknown if there is a cap to the % on the additional bonuses.
Casting a different element -ja spell will erase this effect and start the timer for the element of the new -ja spell which was cast.