The RER train takes about 35-40 minutes to get from Paris to DLRP and the trains run approx every 15 minutes. There is a link to a timetable in The Very Useful Info sticky, follow the section marked Travelling to DLP by Train.
It would be best to aim to be there in time for park opening at 10am so you're looking at catching a train round about 9am so you'll have ample time to breakfast in Paris if you wish.
If you're staying in Paris are you planning on having a Paris Visite card or a Mobilis ticket? If you do make sure you buy one that allows travel between Zones 1 and 5 (DLRP is in Zone 5) although the cheapest way to travel to DLRP is just a straightforward ticket, you'll need one there and one back, and I think these cost between 6 and 7 euros each. It all depends on how many days you're visiting Disney and whether you will be buying any sort of transport tickets for the other days of your holiday.
