I might be wrong but you cannot change how long a sprite will be shown without sending this message several times (according to your needs). Although you can change the frame rate but it can make a sprite look weird.
write_* parameters are (from ./scripting/include/message_const.inc) :
Code:
#define TE_EXPLOSION 3 // Additive sprite, 2 dynamic lights, flickering particles, explosion sound, move vertically 8 pps
// write_byte(TE_EXPLOSION)
// write_coord(position.x)
// write_coord(position.y)
// write_coord(position.z)
// write_short(sprite index)
// write_byte(scale in 0.1's)
// write_byte(framerate)
// write_byte(flags)