I don't think there's a good way in SAMP to 'limit' the time of the Fire Extinguisher spray.
But to be honest, I am sick of you crying mostly because you do not understand how the game actually works.
It takes 3-4 seconds to 'recover' from the fire ext, not 10 as you mentioned. /tazer is similar to this situation. I can rob a cop 50k, and if he is using /tazer and catches me, he will cuff me easily, and get his 50k instantly.
Check this out:
Now you are going to say that /tazer can be dodged but so can the fire ext.
Also even if the fire ext gets removed completely, I can still pwn your ass if I have 100% muscle or so, very easily. No need kung fu, fire ext, whatever.
I am not even going to talk about the fact that using fire ext puts yourself in danger because if the opponent has lag and/or manages to dodge the 'spray' (which is easy, just keep distance), you will die because it takes a while to stop the fire ext animation AND get a gun THEN aim at the opponent, shoot, etc.In my opinion, this looks balanced. But if indeed this gets removed, remove the /tazer too or the cops will be too overpowered.
Peace!