1) On a "normal" day, they give you a 15-min Grace Period.
2) But, this is totally discretionary at the eatery - they could give you none.
3) In busy/packed times, there is no Grace Period.
4) If you are late, the eatery can cancel your ADR.
5) They may or may not let you wait for an open table.
6) If so, there is not time frame - it could be more than an hour or so.
7) Best practice is to be 15-min early.
NOTE: There is NO good excuse for being late for an ADR. Some folks toss up "a broken monorail". Sure, that happens a couple times a month, but it is still no excuse. It is your responsibility to be on time. Being late is rude, and it is disrespectful of the restaurant and the other guests. And, host/hostess have heard all the excuses why people were late. What it boils down to is the folks did not plan their time, or they waited too long to get on a ride.