Hi Sidhantx and welcome to the community.
In principle, yes you can start using Lifeflow immediately with the meditation.
Most people find however that practicing the meditation for a while by itself and becoming comfortable with that first and then starting with LF later on works best for them, otherwise the LF track can be somewhat of a distraction whilst trying to get to grips with meditation.
As for how long, that depends on you. If you're happy with the meditation practice and you feel ready to start the LF tracks, go ahead.
Hugs
Giles