I make ADRs at least four or five hours apart, and that's only if I am booking three TS meals a day, otherwise I try to space them out more then that, since it really is a lot of food, and Disney portions are quite large.
That being said, I like to eat early, so I always get the earliest possible breakfast and lunch ADR possible. Depending on the time of those, I usually book my dinner for the earliest time possible, or six hours after my last meal, whichever is longer.
An example of my planned ADRs for my next trip.
Day One
CRT - 8:00 AM
GF Cafe - 11:45 AM
CA Grill - 5:30 AM
I feel that the time between lunch and dinner is sufficent since it is almost six hours. The breakfast and lunch times are a little too close together for my liking, so I may push back my lunch time when I do finally make my ADRs, but then I'd push back my dinner time too, which would lead to a longer wait to be seated, (also an issue to consider.) I may keep it like this though since I know I'm not really going to CRT for the food. Three of my days are like this, with three meals, so I'll be FULL these nights. I just won't stuff myself silly at breakfast, and won't force myself to clean my plates at lunch and dinner, though I hate wasting food.
Next example, day three's plan.
Tusker House - 8:00/9:00 ADR (Earliest as Possible)
Boma - 5:30 PM
That gives me at least eight hours between breakfast and dinner, this is however the only day I have planned that I am not going to be eating a table service lunch and dinner, so it's also the only day I have more then approximatly six hours between meals.
Last example;
My plan for day seven is what most of my days, (seven out of the eleven full days,) looks like, with a TS lunch and dinner.
Le Cellier - 12:00
Kona Cafe - 5:00
That only gives me five hours between meals. I do like eating early, but in this case I may make a slightly later dinner ADR, though knowing me, I'll just leave it with these times, and try not to over-eat.
So basically, to answer your question, I'd say at least six hours between meals, though I'm going to go against my own advice, and only give myself five hours between some meals.