The OP asked about Metro service on a Sunday, which isn't the same thing as train service at Gare de Lyon.
At Gare de Lyon, it is a good idea to arrive at least 30 minutes in advance to find out where you need to go. Allow much more time if you need to purchase tickets.
Trains and their platforms are announced 20 minutes before departure.
Expect delays and possible cancellations at this time of year.
If your train is cancelled or delayed, you can use your tickets to get on the next one, but won't be assured of a seat.
The Metro runs on a reduced schedule on Sundays and holidays.
If you need to buy a ticket in the machine, plan on 3 minutes.
Finding the right direction is easy - look for the signs with Gare de Lyon listed.
Time to reach the correct platform, 3 minutes.
Depending on which line you are taking, the wait between trains on Sunday would range from 3 to 8 minutes.
If you aren't used to public transportation, need to make an underground connection between lines, and are carrying luggage, I would add 5 minutes or so.