The best thing to do with any shy kitten is to be in their space and ignore them; reading a book or playing on your phone are good ways to pass the time. They will eventually get bored with hiding and come investigate. As tough as it is, continue ignoring them. They need to decide you are harmless, and any movement toward them could be misinterpreted as aggression and set things back. Once they decide you're not a threat, then they will be more open to interacting with you. Ease into it gradually, and once they learn each step is safe, they will work up the courage to take the next one.
As to how long it should take, that is completely dependent on their individual personalities, genetics and how well the breeder socialized them. Some cats are brave and outgoing, and others are shy and reserved, so there is no fixed timeline.
Also, hybrid breeds in particular can have more extreme personalities due to their wild genes. The Asian Leopard Cat is rather shy and anti-social even by cat standards, so it's not surprising that Bengals tend toward the same and often need more time than usual to get comfortable with new humans. The flip side of that is when they do finally warm up to their chosen human (usually only one—and expect your two to pick different humans), they are fiercely loyal, affectionate and protective. When you get there with yours, you will see it was worth the wait. In the meantime, just enjoy their crazy kitten antics!