With assistance, protection, and active planting? 15-20 years.

Assuming humans are gone and nature takes its course?

Centuries. Being an (apparently) unprotected coastline, I'd wager longer still, as young plants would be eroded by weather/water and eaten by animals or just straight up not get enough nutrients.

It's insane just how long it can take some areas to regrow after being cleared (or in this case, probably not existing to begin with)