Never been there, but I found this.... It is served by the TGV, Intercites, Interloire, TER Centre-Val de Loire and Inter-gares Shuttles and is on the train line Paris-Austerlitz <=> Bordeaux-Saint-Jean and Vierzon <=> Saint-Pierre-des-Corps with 4 (plus through lines and sidings) tracks and 2 central platforms.
So I would give yourself probably 5 minutes walking really slow if you are far away from the stairs/escalators/elevators that help you change platforms if your first train is on time, but the hope is that the train actually IS on time. If you're running (because you are late) I would bet you could do it in 3 min or less.