Returns the 'life time' of the particle, how long the particle existed since its creation.
This value will always be between 0 and CLuaParticle:GetDieTime.
It changes automatically as time goes.
It can be manipulated using CLuaParticle:SetLifeTime.
If the life time of the particle will be more than CLuaParticle:GetDieTime, it will be removed.