train stations

Good question!

It depends on a few things:
Do you have your tickets already? If not, you'll want to arrive at least 30-40 minutes early to queue in line unless you have a pin-and-chip credit card that allows you to use the machines. In that case arrive 15-20 minutes early.

If you already have your ticket, make sure you; re at the train station at least 15-20 minutes before your train leaves. Do not arrive any earlier than that because there's always very limited seating in the waiting areas and most of the time the track where your train is leaving isn't updated until right before the train arrives.

And keep checking the board as there are last minute platform changes

In the larger Paris stations, there's often separate sections for "Grandes Lignes" (long distance trains) and "Banlieues" (suburban trains); knowing which one you want will help you find the correct track.

Others have posted details about Gare de Lyon on this thread. There are two different halls, and you need to get to the right one: https://community.ricksteves.com/travel-forum/transportation/gare-lyon-to-basel-onwards-to-lucerne

Usually think about 5 to 10 minutes once familiar with big trainstations in general to my opinion after buying the tickets. It depends how quick you understand the station layout once arrived. In general there are two types of railwaystations: end-of-the-line (like most main railwaystations in Paris) or transit (like RER stations in Paris) or a very few combined (Antwerp, Belgium). First thing to do after buying the ticket is reading the timetables showing also the platform numbers. With the first type station entrance, main concourse and platforms are usually on the same level, the second type they are connected with corridors (with stairs/escalators, sometimes lifts) running perpendicular over or underneath the parallel located platforms/tracks.

Platform/track numbers are clearly shown as well as clocks and boards giving the latest information about departure, delays etc. , listen well to announcements. If no experience with traintravel arrive a bit earlier to get familiar. To my experience buying the ticket can take more time then expected as for instance the ticket machine doesn´t accept your card, you need more time to know how it works ore like last Thursday the ticketbooth closed when it was my turn and had to stay in another line and the person in front had so many questions to ask.

All bets are off at Montparnasse. A mountain guide, breadcrumbs, and a ball of string may help.

Seriously though, it is a super big conglomeration of station. Get there, if you need Montparnasse, a little early.

Google the station and check out the map and configuration before you get there. Lot of helpful info out there to help you plan. Also check with the info booth, they will point you in the right direction.

Thanks very much. this helps.