It is a good 2 days each way minimum. 4 hours Jersey to Honfleur, 6 hours Honfleur to Rouen, not worked out travel time for Rouen to Paris yet.
The tidal part of the river, Honfleur to Rouen is the tricky bit, timing needs to be right on the day to do it in 6 hours. The speed limit for most of it is 28 km/hr, 14 km/hr for the last 40km. The interesting bit is that this is max speed through the water, so if you time it wrong you can easily make that 9 or 10 hours. The time to enter the river at Honfleur is LW+2 to get a fair tide all the way up. Of course, the tide times for the particular day which dictate when you enter the river will also have an effect on whether you fit another leg in before or after that leg. The other thing to bear in mind is that navigation is prohibited in the tidal river between sunset and sunrise.
One option is to pick a day with a LW Le Havre/Honfleur around 10am. Enter the river at 12.00, arriving Rouen at 18.00 for an overnight stop. This could work well for CI'ers specially if we overnighted in Alderney the night before, leaving say 06.00/07.00. We'd need time at Honfleur to group up, refuel etc, before setting off at 12.00.
Admittedly that won't work well for Tommy coming across the Channel so maybe a LW of 06.00/07.00, we all meet up and overnight in Honfleur or Le Havre and get to Rouen mid afternoon. This would then give the option of going further upstream as night nav is allowed further up.
The only trouble with racing up there is we'll miss the unbelievable amount of interesting stuff there is to see on the way, we could easily spend a month getting there if we didn't have jobs to go to