Depends on the continuity, as with most things in comics
Dick is usually depicted as being around 10-12 years old when he becomes Robin (sometimes apparently as young as
eight, but I've actually never seen any real evidence in favor of that), while Bruce is usually in his mid-twenties around that time. So the age gap is around 15 years, give or take. That's how it was in the Golden Age, and how its usually been even going into the Post-COIE era.
The New 52's compressed timeline had Dick becoming Robin at 16, with Bruce being around 26 at the time...so this reduces the age gap to just a decade.
For what its worth, Chris O'Donnell, who played Robin in the Schumacher films, is 11 years younger than Val Kilmer and 9 years younger than George Clooney. So that age gap averages to about a decade. Adam West and Burt Ward on the other hand were 17 years apart.
To sum it up, the age gap is usually somewhere around 10-15 years whichever way you look at it. IMO, its never really been enough to be a ''father-son'' relationship, whereas it makes a lot more sense for it to be a ''mentor-protegee'' or ''big brother-little brother'' kind of relationship. Assuming Pattison's going to be in his late thirties by the time they have a sequel with Robin in it, they'll likely cast an actor who's in his early twenties - with Pattison playing Bruce in his early thirties, and the actor playing Dick maybe in his late teens. I think that makes sense.