Planning ahead - single day in January & reservation timing.

Hi all! I was last in Paris in January 2020 and had a fantastic time. In January 2026 I'm going to London and realized I could take the Eurostar to Paris and see Notre Dame, since I couldn't when I was there before. I've bought my ticket for 23/1/26 and have a 9:30 scheduled arrival and a departure at 18:02 (having looked at the recommendations for getting to Gare du Nord, I'll be aiming to get back to the station about 16:30, since that gives me a little cushion on the 75 minute recommendation).

It looks like the free reservation to Notre Dame isn't required, but I could reserve a few days in advance. Barring any delays, it looks to be about 20 minutes on metro or RER B, so a 10:15/10:30 reservation seems reasonable.

How long should I plan for a visit to Notre Dame? If I have enough time, I'd like to include a trip to Musee d'Orsay as well - there were some permanent exhibit things I missed during my last visit that I'd like to see, but if Notre Dame is likely to take longer than, say, 90 minutes, I can plan for something else, and I'm ok with that.