Just to be clear, multiphoton processes do not change the photon energy of the light (which is what you incorrectly call "the eV of the light"). Multiphoton processes simply use multiple photons to enable processes that require higher energies than are available with a single photon.
In addition, while the pulse duration is a relevant quantity to the probability of multiphoton processes, it is not a determining quantity. What matters is the intensity (or, equivalently, the photon flux density, i.e. the number of photons passing per unit transverse area per unit time). If you don't know the pulse energy and the focal-spot waist, the pulse duration is useless in finding it.
And, as mentioned in a previous answer, if what you want is to observe the sparks and pops of optical breakdown in air, the seed of the process might be multiphoton ionization, but the bulk of the ionization and plasma creation is via collisional ionization in an avalanche process where ionized electrons hit other molecules and knock other electrons out. (Just to be explicit about this: this is not what's normally understood by a multiphoton process.) As with multiphoton ionization, the pulse duration is relevant but it is not the only quantity of interest.
And as for this,
help me by seeing how I'm going about this
we can't help until you say what "this" is and what you actually want to achieve. If all you want is to see pretty sparks then just come out and say so straight-up, instead of continuously beating around the bush with questions that don't actually address what you really want to know.
