Ventilation use

I have a 2x2x48 tent with fan and filter when should I start using it and is 300watt grow lite enough for 2 plants

It should be used throughout the entire grow. You can save some miles on the carbon filter by removing it during veg.
The whole point of ventilation is to keep the old air moving out and fresh air moving in.

I m at seedlings right now should l still use it

Do you like breathing fresh air? So do they.

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Air exchange is important and mechanical strengthening due to air moving the plant around is just as important. The carbon filter is not to be used until flower when your humidity will start to drop (high humidity will kill your filter). That said; as seedlings there is enough air in your small tent to not need an exchange nearly as frequently as a larger plant. A fan in the space to move air around is more than enough. Keep a vent or two open and that will give you enough exchange for now.