
Weeks, with genuinely low risk while it sits — a brush pile on the ground is stable and is not going to hurt anyone. What degrades is not safety but your options and price: after a widespread storm, reputable crews book out for weeks and municipal brush-collection windows close, so the same job gets slower and more expensive a month later. The one thing to check before touching anything is whether a service drop is lying under the pile.
Not a save-or-remove question at all. The trees that dropped it should get one look for hangers and torn attachments, and the debris itself goes one of two ways: chipped on site and returned to you as mulch, or hauled off. Chipping on site is usually both cheaper and better for the beds.