It depends on when you arrive (rush hour or less traffic), and on how you transfer (metro: how much luggage you have? or taxi: how long the waiting queue is?). Whether you have to purchase tickets and have the right currency? Do you know how the Metro works?
All of these “small” things add up to 5 minutes here and there… including reading the bulletin boards to find and get to your departure platform when you get to the Paris Train station you want. That said, it’s wise to leave between at least one to one and a half hours to get to Gare Montparnasse, Gare de Lyon and Gare d’Austerlitz. If you take a taxi during rush hours, the more time you have the better. Gare St Lazare and Gare de l’Est are closer so you’d need a little less time.
The Paris Metro system with a map and instructions
The Paris Regional Express Train Network ( RER ) with a map and instructions