The two places leaves genuinely do harm
First, the lawn. A thick wet mat of leaves left on grass all winter blocks light and air, and you lift it in March to find yellow, slimy patches. Rake the lawn, and use what you take off elsewhere.
Second, diseased material. Leaves from roses with black spot, or anything you suspect of carrying fungal disease, are better out of your own compost cycle than returned to the beds beneath the same plant next spring. The same goes for leaves swept from a pond surface — get them out before they rot in the water. Everything else is fair game.
How long does leaf mould actually take to be usable?
Roughly a year for a rough mulch you can spread on beds, and closer to two for the fine, crumbly texture suitable for mixing into potting compost. Shredding the leaves first, keeping the pile damp and packing it reasonably tightly all speed things up — and the simplest trick is to run two piles side by side, so one is always ready while the other is maturing.
What if I only have a balcony and no pile of leaves at all?
Collect a couple of sacks from a nearby park or a street verge where nobody minds, avoiding anything scooped from busy roadsides or soaked in salt and grit. A single bin bag tucked into a corner will give you enough material by next autumn to top-dress a row of containers, and in the meantime the dry leaves themselves make excellent insulation around pots through the coldest weeks.
