Metro train time Nord to Montparnasse

Here's a link showing about 25 minutes including walking from the official Metro website. If you want to change destination just use the drop down menu. Remember you'll need euros to pay for your metro ticket either from the machines or ticket booth. If you're staying in Paris for a while, purchase a 10 ticket carnet for discounted rides.

You can buy metro tickets from the departure lounge in London, which is what I would do. Lines to buy tickets can be long at GdN and I like to minimize my time in GdN. I know the RATP site says less time, but I would plan on more like 35 to 45 minutes for this trip, especially if you have luggage. Below are the directions that I followed from www.seat61.com and found really useful.

Walk off the Eurostar platform, turn left, and follow the signs for Metro line M4.
Take metro line M4 direct to Montparnasse Bienvenue (follow signs 'M4 direction Mairie de Montrouge').
In contrast to other Paris mainline stations, where the metro platforms are more or less directly underneath the mainline platforms, at Montparnasse it's a 700 metre (750 yard) underground walk from the metro station to the mainline TGV platforms through broad well-lit well-signed subways, with moving walkways for the long bits. Allow plenty of time for the transfer, and if you're not good with longish walks with luggage (even with the moving walkways), consider taking a taxi from Paris Nord to the Gare Montparnasse.

The first response's link to the mass transit system is an essential tool for moving around Paris. I use it all the time when there and suggest practicing now while considering your possible plans. One caveat: Its time estimations are on the optimistic side, at least for me with my chronically befuddled ability to follow signs.