The thing is, "that much screen time" (let's say 15-20 mins) is still more than enough time for The Joker to have an important, impactful role in this film. It's also ample time for The Joker to have a looming presence (even when he's off screen) over various aspects of the film, and the film as a whole. The important and impact of main/supporting characters are not measured in minutes of screen time.
Your example of Hopkins in Silence of the Lambs is a great comparison to make here. Just 16 minutes of screen time allowed Hannibal Lector to be one of the main focal points of that story and undoubtedly the most memorable character/aspect of that film. Despite the character having limited screen time and being somewhat removed from the main plot/conflict of the film (the FBI hunting for the Buffalo Bill killer/kidnapper), Hannibal Lecter still has such a huge presence over the entire story. It also earned Hopkins an Oscar. Another good example would be Darth Vader in SW: A New Hope, who only had roughly 12 minutes of screen time in that first film.
Let's also remember that Suicide Squad is essentially an ensemble, team-up film with a group of central characters, and that The Joker has never really been marketed or billed as the main antagonist/conflict for this film. Compared to the 33 minutes of screen time that The Joker has in TDK (a solo Batman film in which The Joker as the primary antagonist was a huge focal point of the film and was involved in virtually every conflict/event in that story) I'd say that Leto's Joker having roughly half that amount of screen time in Suicide Squad is more than enough for the kind of role he'll have.
Beyond that, we're talking about The Joker here, who is arguably one of the most interesting, unique, flamboyant, charismatic, over-the-top, and menacing fictional characters of all time, so any concerns of him being overshadowed or forgotten among the other aspects of this film are unwarranted. I'd say it's literally impossible for The Joker to NOT have an enjoyable, impactful, and memorable role in Suicide Squad -- whether he has 5 minutes or 50 minutes of screen time.
Either way, though, I don't think any of us should be concerned right now, especially since "my friend on FB counted The Joker's screen time in his head" doesn't really equate to a trustworthy, factual statistic.