Arriving CDG at approx. 0600. How much time should I allow to clear customs, immigration and travel to Gare St. Lazare station?
I think 3 hours should be comfortable, maybe less if you catch the trains without too much wait time and get the "B" express to Gare du Nord, then change to the "E" to GSL.
I would worry about landing late. Where are you going? Do you need to buy an advanced ticket?
The RATP website for the entire region's public transit will give you a step-by-step path to follow
There will be some stepping, that is to say walking. The website's estimated time for the transfer is also a little optimistic, based on a commuter who knows how the system works and doesn't have to pause to translate direction signs.
You need to allow up to one hour to clear immigration/passport control and one hour to travel into Paris on the RER. Take the RER B from CDG to Gare du Nord, walk through the connecting tunnels to the Magenta RER station, and take the RER E one stop to Haussmann - Saint-Lazare. Walk to Gare Saint-Lazare. Do you intend to take the train to either Caen or Bayeux? If so, the first train you could reasonably expect to catch departs at 08:45.
If you want to save money by booking a Prem's discount fare ticket (€20.00) in advance, then play it safe and book the 10:10 train. Prem's tickets are for a specific departure time and are non-exchangeable and non-refundable. If you miss your train, you will be out the price of the ticket and will have to buy a new one at full fare (€39.30). You can book up to three months in advance at tgv-europe.com. Remove the check mark from the "direct trains only" box and choose France as your ticket collection country. If you choose the USA, you will be bumped to the Rail Europe site which doesn't offer discount fares.
I should have included this link which has photos of St-Lazare and other train stations that could help you feel comfortable making the transfer. www.parisbytrain.com