I agree with the previous posters that there is no set amount of time you can be early or late for an ADR. In each case, all you can do is ask and hope for the best. I don't being late often and at most it was 10 or so minute and no problem. I have tried checking in early with mixed results. If it is very busy, they will usually ask you to come back 15 minutes before your ADR time, I specifically remember that happening at 50s PTC. On the flip side, I have gone over an hour early and asked if they could accommodate at other restaurants (Garden Grill comes to mind) and they were able to take me early. It really all depends on the current situation at the restaurant, availability and how far behind schedule they are.
All that being said, I would never plan on being late or overly early or expect to be accommodated.
I do think that if you have extenuating circumstances, i.e. stuck on a broken down monorail, waiting on a bus for an unusual length of time or stuck on a broken down attraction, they will do their best to accommodate.