how much time do I need after landing Charles de Gaulle airport to Saint-Lazare train station

You’d want to get an official taxi from the airport taxi stand, and the ride should take under half an hour (and cost 53 €). It’s generally considered not a good idea to book a non-refundable train ticket for the same day as your flight in, in case of flight delays.

If you are open to a short walk to save some money, Roissybus will take you from
CDG to the west side of Opera, from there it is an easy 2-3 block walk up Rue du
Auber/Rue du Havre to St Lazare. I've had no problems doing it, but it depends on
how much luggage you have and how comfortable you are finding your way after
getting dropped off from a bus in the middle of Paris.

[https://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/public-transport/roissybus]

The bus fare is 16.20 euros, so it's a substantial savings from the taxi, and the bus
is a motor coach (and runs every 15 minutes).

Not necessarily for everybody, but never hurts to have options.

Just a note that the fixed taxi fare is now 55€ (as of February 1).

Since no one has given you a time estimate yet... 4 hours. It may leave you with time to kill at the train station, but you need to make allowances for a late arrival, delays at passport control, picking up any checked luggage, a long line at the taxi queue, or traffic delays during rush hour. There are frequent TER trains, so no advance purchases are really necessary.

Another vote for RoissyBus. I take this almost every time, and I routinely stay near, or leave from Gare Saint-Lazare. The buses leave right outside of arrivals at CDG, and you not only save €, but you don't have to transfer, or go up/down multiple stairs or escalators like taking the train and metro. The only time I took a taxi was when I had to, out of necessity, carry an inordinate amount of luggage. It was similarly simple, just way more costly. I usually buy a cheap ticket with a generous amount of time, and if I have extra time, sit in a cafe across from the train station. If I happen to miss my train (hasn't happened yet), the tickets are cheap enough.

Thank you for all your help