That's a practical question I've wondered about myself when seeing public bins in parks or streets. From my experience and research, most standard urban trash can lids aren't designed to support human weight. Typically, these lids are made from plastic or thin metal and are meant only for covering garbage - not as seating.
Manufacturers rarely specify exact weight limits for lids since sitting on them isn't their intended use. However, based on material strength, I'd estimate most could temporarily hold between 20-50 pounds (9-23 kg) before risking damage. Thicker plastic or metal lids in premium models might handle up to 100 pounds (45 kg) briefly, but there's no guarantee.
The real concern is safety. These lids can crack unexpectedly, potentially causing injury from sharp edges or falls. The hinge mechanisms are particularly vulnerable. I've seen several broken lids in my city that likely failed when someone sat on them.
If you need temporary seating in an urban area, I'd recommend using actual benches or designated seating instead. Public bins serve an important sanitation function, and damaging them creates problems for everyone. The containers themselves are sturdier, but the lids remain the weak point in most designs.
Some cities are now installing more robust street furniture specifically to discourage inappropriate use of trash receptacles. It's always better to use items for their intended purpose - both for your safety and to maintain public amenities for the community.