Plan on stopping every 1 1/2 hours to 2 hours for play time and or snack/meal time. She is going to be sore, cranky, tired, bored, happy, enjoying seeing new stuff, napping at odd times during the day and awake odd hours of the night, etc....
We map out our road trips with McDonalds and parks or school playgrounds planned as stops. She will need physical movement where she can run around and de-stress. Life will be much easier if you do it this way. A DVD lasts about an hour and a half. I would say stop after every movie.
On the other hand: If you leave late at night you can eat dinner, drive for an hour to 2 hours, stop for a late night snack and some play time, then count on her sleeping most of the remaining way. But think about this...how would you like to be strapped on your back to sleep and not allowed to roll around, sleep on your tummy, all those wonderful things we take for granted. If you can lean her seat back quite a bit where it is still safely placed then that will help her to be more comfortable too. If she is not going to sleep you'll have to make more stops too.
This would be my plan if it were me:
Leave home, put in movie.
Let her have lots of fluids and have some safe snacks on hand
Stop for mid morning snack and play time. Spend about half an hour.
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Drive another section. Do games, color, read books, play active things she can really get mentally in to like Dora or other cartoon characters she likes video games. Stop for lunch and bathroom breaks. Play about half an hour here too.
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Plan on nap time for little one, play quiet music on laptop, plan on having ear phones on so driver won't take nap too...LOL. Ive snuggly blanket, have a snuggly available. Drive like crazy so you can make up for time lost playing at McDonalds.
As soon as she starts stirring to wake up look for signs for next food and play area. Maybe already have them listed and highlighted to save time. She'll need some extra play time now. She has refreshed and is fully ready to run to and fro the rest of the day. Expect a struggle when putting her back in the car seat. She knows what to expect now and will want to stay playing.
This would be either an early dinner or late afternoon snack.
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Drive until she is starting to get hungry or restless from sitting too long. She is starting to get sore from sitting in the same position for too long. She needs to eat a full meal here and play. She may go back to sleep if she gets a full tummy. Maybe not but it can't hurt to try. Drive as much as possible until dark.
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This should be the last stop before reaching your destination. She needs a late snack, play time, change into pj's, have drinks ready for bedtime ritual, do calm things with her to get her to go to sleep. If she has had too much sleep during the day she might be the crankiest baby at this time too. If so stop, get out, let her move, let her get tired. Then try again. Let her sleep until you get to your destination.