I started to get some slips on the large chain ring of my road bike after 60 kmiles. This ring was one with symmetrical teeth so I could flip it over which let me continue to use it for another 35 kmiles before it again started to slip when I'd push down hard when starting from a stop. The somewhat more common problem with worn chain ring teeth is 'chain suck' where the chain isn't released cleanly at the bottom of the ring.